Which one?

I am still a newer photographer when it comes to weddings, I use my 70-200 2.8L with my Canon 7D... and 28-135 on a Rebel t3i. So, the question is.... do I spring for a new L lens? Or a full frame 5D? 24-105 f4 or 24-70 f2.8 ($700 price difference) I will only be getting one of these things bc I've already invested so much, but what should I do?

You need something wider.

I'd go either 5D3 or 6D.

A 6D and the 24-105 can be had for less than a 5D3. Pair that with your 7D and your in pretty good shape for a budget set up.

If you're going to continue using your 7D and T3i, I might recommend an EF-S 17-55/2.8 IS USM instead of either of the EF lenses you're considering. The EF-S lens will be excellent on those APS-C cameras. But, I do like using 24-70/2.8 on an APS-C camera for portraiture (I still use the original version, but find it a bit "too short" for portraits on full frame... On FF I use 85/1.8 and 135/2 primes.)

IMO, there really isn't much practical difference between the 28-135 you've already got and the original 24-105/4L. Image quality, focus speed, close focusing ability, stabilization and most performance aspects are very similar. The 24-105 goes slightly wider, but has more vignetting. The 28-135 goes a little longer, but gets slightly soft racked out to 135mm (stop down to f8 or back off to 105mm and it sharpens up). The 24-105 has a non-variable f4 apertures (versus an f3.5-5.6) and seems better sealed for dust and weather resistance (not usually an issue at weddings), but doesn't seem any more durable in spite of being an "L". Both lenses have their "foibles": Theyboth tend to have "zoom creep" and both can have AF/aperture issues that seem related to the internal flex cables.)

The new 24-105L II is said to only be modestly better in image quality... Too early to say if it's going to be more durable and is better sealed, but it supposedly has higher performance image stabilization. It's also just as big and nearly as heavy as a 24-70/2.8 II (although the 24-105 uses 77mm filters and the 24-70 II uses 82mm).