Lens Dilemma

I have some money from my grandparents coming about halfway through november (either $1700 or $1800) and I have decided to buy a proper Telephoto, after I invest in some pretty high dollar software for college as well as for the rest of my natural life.

My Sigma 150mm f/2.8 Macro with the 1.4 TC does a decent job at being a telephoto lens, but quite frankly, this setup has some issues. Mainly, the fact that the HSM is rather slow (compared to the USM im used to) and there is no stabilization on this lens, which is a problem when shooting handheld in low light situations with the TC equipped.

Now at first, I thought about going with the Canon 100-400mm lens, but it is a bit pricey. That is when I started looking for similar offerings from other companies. Sadly, i did not find anything that I found to be a suitable alternative. Remembering that PopPhoto.com has a list of approved photography equipment resellers, I decided to look through their list of websites next. What I found, seems like a pretty good deal.

On several sites in the Check Rated list, I found the 300mm f/4L IS lens, as well as the 400mm f/5.6L lens. Quite frankly, I am tempted to go with the 300mm lens mainly because it is almost $400 cheaper than the 100-400mm, with a slightly larger aperture. And from what I have heard, primes are better than zooms; however, I do not fully understand how this concept works (my guess is that because primes have no moving internal lenses so there is less chance for diffraction).

If I chose the 300mm, I could simply get a 1.4 TC to use with the prime to get past the 400mm mark of the other two lenses. However, that would give me the f/5.6 aperture of the 400mm prime lens, so it would make sense to simply go with that, since it is about the same price as the 300mm lens. However, the 400mm lens has no IS system, and therefore I would be having the same issue with hand-holding that I am currently having with my Sigma.

At this point I'm not sure what to do, but I am leaning towards the 300mm and 1.4 TC option. If I do go this route I will end up having to get a Camera Backpack, which would actually be pretty cool. And just how good is this lens compared to the 100-400mm? Better, Worse, or just as good?

I would personally go for the 300mm + 1.4tc option, as this setup would give you 2 focal lengths (with, or without the TC), whereas the 400mm would only give you a single focal length. Also the 300mm (as you point out) has IS which helps when shooting distant things.