Go cheap and get the nifty 50 (50 f/1.8 II). It's so cheap it won't strap you and stopped down it's razor sharp. The 50 f/1.4 is better, and more expensive, but still cheap ($360ish). When I first bought my 5D Mark II a few years ago I had the 24-105L kit lens and picked up a quick nifty fifty. I didn't regret it. Of course as my photography expanded, my lens selection got significantly more expensive. But here you have nothing to lose. Purchasing the 50L you are doing a serious investment. You can get many other primes that are very sharp stopped down for that price. If you need 50mm wider than f/2, the 50L makes sense. But if you're shooting above f/2.8, any of the primes at 50 f/1.4 or f/1.8, 85 f/1.8, 100 f/2 are great. My recommendation is to not go expensive first. Of course, there's always the new 40mm pancake lens.