Borrow another interface if you can and record the same guitar part with both of them. Have someone there that can play back the samples in a blind test. 75% chance you won't hear much of a difference. Now build a small mix (drums, bass, your guitar tracks). Have your helper pan them left and right. 95% chance you won't be able to tell a difference, or at least enough of one to make it worth spending more money on another interface.
Now take your extra cash and buy more room treatment and take your girl out to dinner. Both of these things will provide much more return on investment than a new interface.
I had a UX2, a pod and a PreSonus 1818VSL. Which interface I used to record guitars made about 0.0007% of a difference in the mix, and I could eq that difference out pretty quickly.
Andy once had me send him a clip of my voice recorded through either my UX2 or my Pod X3 (can't remember now which) that I was bitching about being low quality, tweaked it and sent it back to me with a clip of Warrell Dane singing the same thing tracked through some sweet studio gear of MUCH higher quality. He made both of them sound pretty fucking sweet. Showed me straight away that pre's are fun to spend money on if you have disposable income, but there is such a minimal difference in pre's from reputable companies (Line 6, PreSonus, Mackie, M-Audio, etc.) and going from them up to the big boys gets you MAYBE a TINY bit of clarity/detail and SOMETIMES a little less noise, but really nothing that jumps out of you in a mix. Those two clips from Andy really took away my urge to spend more money on gear and spend more time recording and mixing. Saved me a lot of cash over the years.
That is my opinion anyway, YMMV. Just food for thought. Whatever you choose to do, I wish you the best of luck with your music!