I've got all 5 seasons on R2 with the movie boxset and the original 'The Gathering/In The Beginning' on R1. I've also got Crusade on R2.
I would say if you can get them all in R1 then do so because:
Season 1 Boxset has some aspect ratio chopping and changing on the last episode of Disc 2 if I remember correctly.
Season 5 Boxset: the episode 'The Fall Of Centauri Prime' has the whole prologue section (the bit before the beginning credits) missing
and the episode 'The Corps is Mother, The Corps is Father' doesn't have the original credits that the show was aired with. What really angers me was that the R2 boxset was released some 10 months after the R1 boxset and Warner Bros still messed up!
Very sloppy.
The R1 boxsets are not blessed with these "features" as the R2 boxsets are.
Also the R2 B5 movie boxset is only 3 discs: Third Space / Call To Arms / River Of Souls, whereas the equivalent R1 movie collection is 5 discs: The Gathering / In The Beginning / Thirdspace / River Of Souls / A Call To Arms. The Gathering and In the Beginning contain extras that the original R2 (or original R1) releases don't have.
*EDIT* Sorry, you've hit a bit of a sore point as you can probably tell.
S.