Very tough choice.
Both have been known to be a little hard to get into, in early S1, but, both become absolutely fabulous Series in my opinion. B5, and Farscape plus DS9 are my 3 all time favorite Series.
Farscape has far more comedy, and they really twist some SciFi tropes on their heads. Babylon 5, is a more serious take (Though Farscape certainly does take things seriously)
If you like Ben Browder's wise cracks in SG1, you'd probably be better off going with Farscape first, because Cameron Mitchell's personality is pretty similar to his Farscape character John Crichton. Buffy had alot of these same type culture wise crack references, and of course Col O'Neill was doing it on Stargate SG1 before Ben Browder was
Babylon 5 is just a sight notch above DS9 and Farscape, in my opinion, but, your mileage may vary.
Alot of the acting portrayals in Babylon 5 are more formal and Shakespearian in nature, which doesn't work for some people.
I own all 3 on DVD (My first 3 DVD Series Purchases, actually) and I would definitely find my collection lacking without any of the 3 being in it.