I would go with the gtx 550ti for the following reasons:
-uses less power
-can use in SLI down the road if your motherboard allows for it
-shorter in length
Now, if you're not interesting in DX11 gaming, you do not care about power consumption, and your case is long enough, then the gtx 280 is a much more powerful and fitting card. I'm not quite sure where you're going to get either one for $100 though because the cheapest ones I can find are at least $120 w/out shipping or tax.
If you're up to looking at a different option, I would actually go for the Radeon HD 6850 as it is a sweet spot for price/performance (and power consumption/heat). It also performs better in pretty much every aspect than the gtx 550 ti. Looking on NewEgg.com - the price for the 6850 and the 550 ti are within $25 of each other same.