I once read a comment to the effect of ‘you can’t discard trump losers’, suggesting that it is better to have the stronger suit as trump. It seems to me that one should prefer the weaker suit as trumps on the basis that weak trump cards can still win tricks via ruffing whereas weak side suit cards always lose. That said, I’ve seen hands where I wished one was the other and vice versa. So, the best thing I’ve found to do is not worry about it and just let the cards fall where they may. It’d be better, of course, to figure out during the bidding which suit should be trump and which should be the side suit and bid accordingly, but I’ve no idea how one would go about doing that considering that the exactly which cards held by partner are unknown and the exact number of cards held is often unknown too.