Hi Rik,
I would recommend holding on to your dough. I have used the 75-300mm quite a bit since my girlfriend prefers it to my 70-200mm f/2.8L IS due to the weight factor. Obviously the 75-300 weighs a heck of a lot less.
Optically, though, I would say it's not a very good upgrade from your current glass. 100mm sounds like a lot, but between 200 and 300mm, it's really not a monsterous difference... I'd say it's more of a framing difference than a resolution difference. In other words, you could probably crop a sharp 200mm image and blow it up and have it look close to the 300mm image from the 75-300mm lens. The 75-300mm Canon is very soft at its larger apertures, though it's okay from f/8 to f/16 or so. You'll also see large amounts of purple fringing on bright objects (some call it chromatic abberation). Bokeh (the quality of out-of-focus objects) is pretty nasty looking.
Yes, I'm obviously not a big fan of the lens, but I'm not a lens snob. It's not the price, construction or lack of red stripe that makes this a poor lens. Just image quality. (The part-metal barrel and metal mount construction is actually quite good for a consumer lens.) Even my girlfriend, who has literally just started shooting with a DSLR, noticed how iffy the images looked. I'm not talking about some slight measurebator difference that you need to compare 100% crops to see.
If you save/invest that money you were going to spend on the 75-300mm and wait for something like the 70-200mm f/4.0 or a good Sigma (Sigma has some very good tele-zooms... unfortunately I can't tell you which ones they are since I don't have one), I think you'd be happier than ending up with two so-so zooms with similar ranges and very low resale value.