Nice thing about a condo (store-bought cages are way too small; you can build a huge NIC condo for the same price as one) is that you can get extra space into the same "footprint" by having levels. Also, in the wild, bunnies prefer high ground so they've got the best view of any approaching predators - mine love to camp out on the higher levels of their condo.
Height depends on the rabbit - my Holland lop is a little loaf and pretty much any pen will hold her. My lionhead has never jumped out of a 24'' pen without something to stand on, but that's probably only because she threatened to so badly that I upgraded to taller pens. If you want to be able to give your rabbit(s) big toys, cardboard boxes, etc. then you need a taller pen because they can push things to the side of the pen and use them to climb out.
X-pens are technically cheaper, but require more floor space to achieve the same actual space. With a pen that's got a total length of 16', you can use screw eyes and zip ties to connect it to the walls to get an 8' x 8' square where two of the walls are actual walls and two are playpen panels - that would achieve the 60 square foot minimum for a rabbit or pair that doesn't get 4-5h outside the cage each day. If you don't use walls to some degree, you'll end up with a 4' x 4' square... which is a smidge small without levels if your bunny spends a lot of time in there.
To make the most of your floor space and give your bunny more variety, I highly recommend a condo - you can build a basic condo (for example, 2 grids wide x 4 long x 2 high condo with a partial second level) for under $60. That said, some people go with just the x-pen and it CAN be a viable cage replacement - in the end, it comes down to personal preference and your own needs.
I don't have a pic handy of my set-up here, which is a 2 grid x 4 grid x 5 grid high NIC condo and a pen to create about 64 square feet of space (including the condo's footprint). At my house in San Antonio, I also used a condo and pen set-up, which took up most of my living room
For pens 24'' or 30'' tall, the best deal I've seen is
this one... if you want something taller than that, the best deal I've seen is
this one. I've never actually bought the Amazon one, but I've got both 36'' and 48'' versions of the second pen (sadly, mine are stupid gold because silver wasn't available back then
) - I'll vouch my a** off for the Precision Pet pens, as they're sturdy and durable as hell. Between humans, bunnies and cats, the pens have taken a ton of abuse. 24'' is a cake-walk for a human to step over and 30'' (well, 29'') was a little inconvenient for me (I'm 5'4''), so lack of a door shouldn't matter much... but if you go taller, you'd really appreciate the built-in doors on the Precision Pet pen!
If you want a little more info on NIC cages (tutorials, some tips on getting good prices for the materials, etc.), I just made a
huge post in another thread about them that has lots of links/tips.