An "L" is not necessarily "better" glass. Depends upon how you define "better". Some "lesser" lenses can match or come darn close to the IQ.... most will not be sealed as well or have the heftier metal construction so may not be as durable, but also won't be as heavy and might balance better on an XSi.
By definition, no EF-S lens can ever be an L. Canon defines an L series lens as:
1. Must be top quality sesign, materials and build (sort of subjective criteria there).
2. Must use "exotic" glass (fluorite or ED, UD, aspherical, etc.)
3. Must be fully compatible with all EOS cameras past, present, and future.
The last one is the problem... EF-S lenses are only usable on 1.6X crop cameras beginning with the DRebel and 20D or later. So no EF-S lens will ever get a red stripe painted on it or the L designation, unless Canon changes their criteria.
I really wouldn't worry about buying an EF-S lens now, if a FF camera is a "couple years in the future". You can always sell the lens later... Probably won't see much depreciation, might even make money on it. I have several lenses in my kit that I could sell for significantly more now, than I paid for them used a few years ago.
Besides, why do you want a FF camera? All things considered, for a lot of folks, there's limited reason to get one these days.... crop cameras are close to matching the IQ and are just going to keep getting better.... and are cheaper, lighter, smaller and in many cases allow you to use cheaper, lighter, smaller lenses.
Do yourself a favor and check out this review of the 17-55mm
. Note that it uses 3 aspherical and 2 UD elements. For comparison, I know the 16-35/2.8 uses 3 aspherical, am not sure how many UD. Yes, the L-series is better sealed against dust and such... and it's made of more metal, less plastic... but it's bigger and heavier, has considerably less focal length range. Note in the review, that the 17-55 is "considerably" sharper in the corners and edges, too.
The 17-55 would be a considerable upgrade to your 18-55... especially if that's the earlier non-IS model of the kit lens.
You've already got a great portrait, low light lens in the 50/1.4. If you need wider or longer, add a 28/1.8 or 85/1.8 (or Sigma 30/1.4, 85/1.4 if you want heavier, more expensive lenses).
If you need wider, Canon 10-22 or Tokina 12-24 are good choices. I think the Tokina is one of the best values... it's very "L-like", yet it's about $100 US cheaper than the 11-16/2.8 and around $300 US cheaper than the Canon. The Canon slightly edges it out for flare control. Neither of these lenses are particularly fast... But, honestly, you are far more likely to be stopping down a wide angle lens, than needing f2.8 on it.
If you get these, I have a suspicion you'll be looking at a 70-200/4 or even f2.8 to replace the 55-250!
