I have the new HL5 and I've read lots about the old HL5 from Chinese forums. My style is two-winged looper with close up touch shots or pushes to create openings. I BH flick every so often, but win most of my points from touchplays, FH flicks, and FH loops.
1. Excellent for loops - because most of yours shots follow an arc-like trajectory, if you like hitting fast flat shots, the ball will go into the net or go out. So you have to always brush the ball both for forehand and backhand in order to ensure it goes over. This applies for both the old and new HL5. I found this to be difficult for my BH at first since I transitioned from a butterfly ALC blade (Viscaria) which followed a more linear pattern for my shots. But after changing my style to more of a brush-like stroke, the ball goes on the table easily.
2. New HL5 has a lower arc than the old HL5 which is designed to be more aggressive but at the same time means it's easier to make mistakes, especially if you using attacking rubbers like Hurricanes. If you are using tenergy 05 for your FH, it shouldn't be much of a problem. I heard the difference in arc height isn't very much.
3. Both Blades are head heavy - If you are adopting a Chinese playstyle where you do big fast swings and use heavy chinese rubbers on your FH like Hurricane, you will get tired easily. You'll tire more easily with the Old HL5 since it's head size averages 161-163 x 151-152, so there's more rubber on the blade vs the New HL5 which is 156-158 x 150-151. The New HL5 allows you to reset more quickly because of the supposedly lower weight. It also allows for better conversion/transition between BH and FH which a lot of players from the Chinese forums found troubling with the old design.
4. Slightly less rotation on the new HL5 compared to the old HL5 - this is due to the reduced head size.
5. Handle size for the new HL5 is slightly bigger and more comfortable than the old HL5 - Not sure how true this is since there's many variations of the old HL5 as well. For instance 163 x 151 to 161 x 151 head size. Some say that for the 2016 batch, they were making bigger handle sizes. In my honest opinion, I felt the handle size to be just right since it allows a lot of flexible for the wrist to move (which at the same time may not be preferred by a lot of players). The circumference of the bottom is about the same as a butterfly blade but the middle portion circumference is less by a few mm which may feel uncomfortable for a few players.
6. Backhand is much better for the new HL5 vs the old HL5 - This is probably due to the reduce head size even though the old one had a larger sweet spot. That seems to be the consensus for most players on the Chinese forums.
7. Touchplay - varies a lot depending on the rubbers you use. It's better to use rubbers with a softer sponge rather than hard sponge, otherwise your ball will plop up high and be easy to kill. I used H3N which had really good touchplay and then switched to the H3 Provincial (designed for plastic ball) which is even better and has more power and spin. BH - Tenergy 05/05-Fx, Andro Rasant Grip/Powergrip, DHS Skyline 3-60 (37 degrees - designed for plastic ball) offer some good touchplay. I personally have not tried 05-Fx or 3-60, but i heard it works.
8. Smaller head size for the new HL5 vs old HL5 - as stated above, the new one measures around 150 x 156-158. This allows for reduced weight overall, quick transition between FH and BH, quick resets (which is needed because the plastic ball slows down a lot of shots), and better BH. Some players say that the flex of the racket is reduced because of the smaller head size which reduces dwell and spin. The reduced spin I can agree on, but the dwell I'm not so sure since the ply is limba, so I still feel the dwell. I never played with the old HL5 so I wouldn't know, but this is all THEORY. Blade has plenty of dwell and it can also vary with whatever rubber you use.
9. Flicks - BH and FH flicks were easy to implement. BH flicks were easy. FH flicks were hard initially because I was adjust to the head heaviness of the blade, but once I adjusted, they became easy to do.
10. Feel - New HL5 is said to have a more muffled feel compared to the old HL5 where the feel is more clear (whatever that means). However, if you play with either for a long period of time, it's supposed to become inconsequential.
11. Power - might be reduced or be the same between the old and the new. My new one was 90-91g which was still on the heavy side. I've heard players getting 87-88. More weight is supposed to increase feel and add a bit more power in your shots. But it'll slow down your swings.
12. Strokes - low power strokes can be difficult to play with the blade. You have to hit high quality shots. There were times I was fooling around or did improper strokes. The ball just dies - ie. hits the table on my side or hits the net. Medium power to high power strokes is where this blade shines, especially when you loop.
13. Control - Very hard to control initially. But after some adjustments, I got the hang of it. If you don't have proper stroke fundamentals, it can be very hard to play with. I've lent my blade to a few of my beginner friends, and they couldn't even get the ball on the other side.
Current setup: H3 Provincial (black) 2.1 on FH and Rasant Grip (red) 2.1 on BH.
-This build allows me to loop low throw angle balls and execute short and low touchplay shots. Only downside is, both rubbers are heavy, so you can get tired very easily. Still working on building my stamina ever since the switch to Rasant Grip. This build also allowed me to hit flat shots when the ball is high.
-Old setups - Tenergy 05 on BH. Excellent touchplay and flicks. BH loops can be very challenging. This rubber excels in mid distance BH loops. But close to the table loops just fly out. You can try closing the angle of your racket more, but it might feel unnatural or you can close the angle of your racket as you are following through (which can be very difficult to do). Flat shots, especially against high balls tend to fly out. I twiddled the BH to my FH and found the tenergy 05 to be easy to use. Rakza 7 on my BH was a nightmare. The BH loops fly out a lot. And the touchplay is too high, making you get killed easily. BH flicks were very easy though and the feel was good.
In summary:
If you are FH dominant and want more rotation on your ball and don't mind the smaller handle- go with the Old HL5.
If you are more balanced and prefer a slightly bigger and more comfortable handle - go with the new HL5.