I, too, recently bought the PSB T55s, in my case from Saturday Audio, to serve as front satellites in a 7.1 system. Must say, after much fooling around, fits and starts, trying to get good results from Audessey calibration with my NAD T775 HD A/V receiver, with 2 subwoofers, going back to the 7.1. configuration, with just one subwoofer, has enabled me to get to that sweet spot we're all looking for.
Like you, I waffled between Image C-40 and C-60. I was upgrading from a PSB Alpha C as center, which performed admirably for more than 15 years for me. In short, I opted for this (older series) Image system: C-60 Center (set the crossover at 60Hz; I tried 80 Hz and got muddled movie dialogue), T55s as fronts, Image S50s (bipolars) as surrounds, and left-over 15-year-old PSB Alpha Cs as L & R backs. Anchoring this system: a new HD8 compact PSB subwoofer. I just ran last night the BluRay version of Master and Commander, which has a marvelously challenging soundtrack, recorded in DTS Master. My room is about 18 X 14, and fronts and center are 10 feet away in front. TV is 58" Samsung plasma, used strictly as a monitor. I never turn on internal speakers or use its TV tuner. In short, the sound quality listening to this (very familiar) movie sound track was just incredible. And the lip sync issues I initially encountered trying to fine-tune a two-subwoofer system with the subs on opposite ends of my listening room were gone. I think I've got it after 3 weeks of fooling around with Audessey and subsequent Small/Large and speaker crossover settings in the NAD. Audessey's CTO runs an extremely helpful blog on bass management with home A/V systems on their Web site. I recommend it highly.
Bloviating....To return to your question: Smaller room, go with the C-40; larger room, you won't be disappointed with the C-60, I guarantee. And for price now being offered, this PSB Image system I've put together, when running films at 24 FPS video, and Dolby True HD (or whatever it calls itself) or DTS Master, these "lossless" sound technologies, I just don't have anything else to quibble about. The sound quality is just breathtaking, and the video on a wide screen is a good match.
End of gush.