You should buy one, then the other when you can afford it. I don't think you can make a bad choice, but I'll lean slightly towards Farscape. Along with being dramatic, epic, compelling, and all that, it's also funny.
I think Babylon-5 while slower is written far more intelligently, but Farscape, its got lovely sets, great actors, beautiful CGI, but its really far away in the cosmos and escapism with that gothic, edgy, sexy, slick, funny feel to it. Don't get me wrong I love both shows but for different reasons, I just thought the plot and subtle story arcs in Babylon-5 were better structured. B5 might visually look a little dated but you got to give JMS credit, he wrote the whole thing himself, it was the only flagship program to challenge Trek when Trek was in its prime and JMS wrote over 18 episodes per season by himself...something totally unheard of today