By having to choose between the two you're already in a bad situation, as every flaw of whichever concert you see will ultimately be "Yet ANOTHER reason why I should have gone to Feeder/Built to Spill..."
By walking into their respective venues, you're already holding them to a high standard, as they have to justify your choice of going there by being MUCH better than the other show. With this mindset, it will be an inevitable disappointment.
With this in mind, you enjoy both bands, and both have their flaws, so instead of weighing them both by their scales and, more notably, the opinions of strangers online, just go to the show that you think you'd enjoy more, not which one you think deserves your presence more.