I would assume that Borderlands will be a competent PC game. I always thought of BL1 and BL2 as PC first games even when I was originally playing them on PS3. I picked them both up again when I built my PC and I was happy with them. They had lots of nice PC settings you don't see in crappy console ports and they both run great on my machine.
If I was purchasing based on which I think will be the better PC port I would pick Borderlands. GTA IV didn't get a lot of praise for its PC port and it was still choppy on my PC that was built almost 5 years after it came out. Hopefully they put a little more effort into it this time.
If I was purchasing based on which I thought would be the better game it would be GTA V. I love the Borderlands series but GTA V was something special. There is a reason it took them 5 years to develop it. The pre-sequel feels like a weird spinoff that may or may not live up to the first two games.
Either way, I would just wait for reviews. There isn't much reason to pre-order a digital download PC game. If the pre-sequel turns out to be a crappy game the choice becomes much easier.
