I can back up rfusca's advice with a practical example. I went through a similar dilemma, looking for a short telephoto to shoot people at events where flash is not an option. My choice was between the newly released Canon 100 f/2.8L macro with a four stop stabiliser and the 'king of low light' Canon 85 f/1.2L. The 100 offers four stops of stabilization. From the dpreview test it can deliver 3 stops stabilisation on a regular basis. So taking the 1/f shutter speed rule, and doubling it for safety, without stabilisation I could shoot at 1/200s. Adding three stops stabilisation gets me to 1/25s with a decent margin of error.

The 85 f/1.2L has no stabilisation but is 2.5 stops faster, but I'd have to shoot it at 1/170s, and lead to less light than the 100. That and the added DoF the 100 gives to cover slight focus errors lead me to get the 100.

However after a while of using this lens I found that people just move too much for 1/25s. I moved to 1/50s and still found movement a problem. I had to work really hard to time shots, and often the ones with movement were the most expressive. So I ended up buying* the 85 f/1.2L anyway (I kept the 100 for macro and still subjects)

One stop is a bit different to three stops but the same principals apply, if you want to shoot people, especially up close, there's no substitute for shutter speed.

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*did I say 'buy'? I meant I invested in this lens. Given the interest rates at the moment I'll probably make more on the lens in the long run!

I can back up rfusca's advice with a practical example. I went through a similar dilemma, looking for a short telephoto to shoot people at events where flash is not an option. My choice was between the newly released Canon 100 f/2.8L macro with a four stop stabiliser and the 'king of low light' Canon 85 f/1.2L. The 100 offers four stops of stabilization. From the dpreview test it can deliver 3 stops stabilisation on a regular basis. So taking the 1/f shutter speed rule, and doubling it for safety, without stabilisation I could shoot at 1/200s. Adding three stops stabilisation gets me to 1/25s with a decent margin of error.

The 85 f/1.2L has no stabilisation but is 2.5 stops faster, but I'd have to shoot it at 1/170s, and ultimately end up with less light than the 100. In the end the added DoF the 100 gives to cover slight focus errors (not to mention weather sealing and macro ability) lead me to get the 100.

However after a while of using this lens I found that people just move too much for 1/25s. I moved to 1/50s and still found movement a problem. I had to work really hard to time shots, and often the ones with movement were the most expressive. So I ended up buying* the 85 f/1.2L anyway (I kept the 100 for macro and still subjects).

One stop is a bit different to three stops but the same principals apply, if you want to shoot people, especially up close, there's no substitute for shutter speed.

-

*did I say 'buy'? I meant I invested in this lens. Given the interest rates at the moment I'll probably make more on the lens in the long run!