Really depends what you're looking for. Both shows are very good indeed, both shows pushed the envelope for tv sci-fi and featured some unforgettable characters...but they're also fundamentally different animals.
First off, a disclaimer: B5 is not "like Star Trek", at all. Some superficial (and purportedly not coincidental) similarities to DS9, but that's as far as it goes. It's very much it's own show and would often (sometimes pointedly) approach things that Trek wouldn't touch, or address them from a totally different angle.
Now that that's out of the way, I love Farscape: is a fun, colourful, often comical adventure series that was unjustly cut down before it's time. B5 on the other hand is a complete story spanning five seasons, with a definite beginning middle and end. In terms of storytelling I'd say it's closer to Firefly than BSG in that while there is strong epsiode-to-episode continuity, it wasn't totally serialised.
Farcape was more or less made up as they went along. Going in they had onle a vague idea of which way the show was heading and so, it meanders quite a bit. B5 was planned from the get go and for the most part, things went according to that plan. Of course it helped that the plan included the possibilities of course correction, cast changes and thinking up a better idea in the intervening time so it comes off as fairly seamless.
B5 got to finish it's story. When you see the last episode, you're not left wanting more (in a good way.) Farscape suddenly cancelled and the last episode leaves you feeling like the rug just got pulled out from under you, then rolled up and used to beat you about the head. Sure, they got a chance to tie up some loose ends later on with a mini-series, but that was a very wonky affair.
I suppose if it comes down to money, I say go with Farscape because it only has 4 seasons and even then you can probably skip most of the first season and you won't miss much (think early TNG in terms of inconsistent direction and quality of writing.) It doesn't really get good until a certain pale chick turns up. While B5 also had some first season growing pains, there's a lot there that lays the ground work and sets up some things that won't fully pay off for several season--Indeed, a surprising amount of it you really only notice in a second viewing--So basically, if you get B5, you'll be hooked and want to buy all five sets.
If you get Farscape, you'll have fun and might pick up the other three. Getting hooked on Farscape is cheaper, so long as you can deal with a lack of a definitive ending.