I'm not using either of them at this point (somehow I've just missed buying those) but still have an observation possibly of use.
I've been shooting for 50 years, literally and grew up on the dying glory of rangefinders and the hey day of the Nikon F, when primes were the rule and only toward the end were the early zooms coming along. I gave up the trade as a profession in the early 70s, but continued to shoot and somewhat revived the old interests to the point of driving my wife crazy.
A burglary forced me to give up the Nikons and for a long time I shot Olympus OMs, then I jumped into digital with the 20D and immediately began adding zooms, etc. Moved now through all three 5D versions, currently on 5D3, plus a smattering of other odd bits, e.g. a Leica M9-p which I enjoy for its sheer manual backwardness.
Probably six or seven years ago, I got a hint of my internal dissatisfaction with zooms, even the good solid L zooms. There are a couple of issues. One is that Zooms have slightly different characteristics that sometimes surface at not necessarily good times, i.e. the odd color cast reflection when shot into the light at some angles. The quality of the good prime will be better than a good zoom, perhaps not enoughto be noticed. But there's also something that it does to the shooting mentality. It's a bit easy to just use the zoom to frame an image rather than doing the obvious of moving the camera and shooter.
It's a mindset, but I also find myself on occasion using the crutch of the zoom to justify shooting some frames -- not hard because I shoot heavily anyway. There is a bit of an edge, IMHO, to the added minor discipline of using the 135 prime instead of the one size fits all. I cover that range with a 70-300 and a 55-250 (latter on aps-c) but if I were in your shoes, I'd lean toward the 135.
I tend to think that there are core focal lengths which just work better as primes, e.g. the 50, the 75-90 length, the 135 for sure and all other things being equal, I'd rather use a 300 prime than the zoom. For what it's worth, that's probably true of all the long focal lengths, it just isn't practical for most of us. I've got a Sigma Bigma which covers the longer shots well enough. If I were doing wildlife, give me the prime.
As an aside, I have both the 15mm and 6-15 fisheyes. As a rule, I prefer the 15 prime. The other is a great lens, weights a bit more and is a bit more versatile in the bag. Working in a more or less known environment, being able to control for the prime, especially given its optics, pushes me that way.
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