Save up for an L prime or buy a third party brand?

I shoot with a t1i and am looking to soon buy a new lens. The ones I have are in my signature, but I'm so conflicted between my few options. Since my 18-135 already has a good range, I figured that it'd be a good travel lens, so I decided that I want a prime. I was looking into a 35mm L because of the shallow depth of field, but am having a hard time choosing between that and the 24-70 L. However, I feel that although the glass is nicer, the range is covered in my 18-135. Would that be a reason to not buy the lens, or is it THAT much better? I'm still in high school and am by no means a professional; is it worth saving up for an L lens or should I look into a third party lens, such as the Sigma 30mm? I also have a 35mm Canon Elan, so I prefer getting a lens with an EF mount. Thanks!

What will you be shooting primarily?

Many out there love the Sigma 30mm, I think it will save you money and it will "feel" like a 50mm on a full frame. Lots has to do with what it is you shoot most.

People/food, and occasionally bugs, but I have the 100mm for that

The Sigma is supposed to be great, unless you plan on moving to a 5D pretty soon I'd say it is worth looking into. The 17-55IS is also probably a better lens for your camera than the 24-70 as the 17mm end is going to be more useful than the 70mm end of the 24-70, it has IS, and it is much lighter. Have you thought of getting a telephoto lens? You can get a 70-200 f4L IS for less, or any number of other fantastic lenses.

If I do upgrade to fullframe (which I might sometime during college), would the Sigma 30mm / 17-55IS vignette? I would imagine that the latter would at 17mm, and I want a lens that would be useful for both a crop and fullframe/35mm camera.

I have considered a telephoto lens, but I don't think they would be much use for me, and 135mm is already enough reach for now

So do you want a prime or a zoom? Your kinda all over the board why don't you give some thought to what you feel you lacking.

They won't even mount so that may be something to consider but I would get what you want/need now. Lenses can always be sold.

I think you ought to look at the Zeiss 35mm f/2 ZE. Nice FOV on your T1i and works as a great WA street lens for your Elan.