XIOM and Donic both make good quality rubbers so, given that this is your first custom bat (have I understood that correctly?) I wouldn't be too fussy. Just ask your friend at the club what he has that's in the 42 - 45 degree hardness range and go ahead with those.
For XIOM rubbers those will usually be the ones that have "Euro" in the name. For example, XIOM Omega V Euro (45 degrees) and XIOM Omega VII Euro (42.5 degrees). The exceptions would be the Vega Asia DF (42.5 degrees) and the Vega Japan (45 degrees).
For Donic rubbers look out for the numbers in the name. For example Bluefire M2 (45 degrees), M3 (40 degrees), Bluefire JP2 (45 degrees), Bluefire JP3 (42.5 degrees), and Bluestorm Z3 (42.5) are all in the range you're looking for.
It's not going to make a huge difference to you whether you go with Donic or XIOM. At least, not at this stage. My personal opinion is that Donic rubbers seem have edge for the first little while and then their performance fades a bit, whereas XIOM rubbers take a bit of 'playing in' but then they seem to last longer.
If pushed I would probably suggest the XIOM Vega Japan or Asia DF (or, if you want a really soft BH, the Vega Euro DF at 37.5 degrees).