I have tried a few and to be honest I don't see a great deal of variation between them. Right now I have one that is variable speed and the other fixed speed, I don't remember ever altering the speed so that for me is an unnecessary feature. Dust collection with the bags is almost universally pointless, but if you can find/make an actual adapter to a shop vac then you can get good performance. The only problem with a hose attached though is that it significantly hinders the ease of use.
I have considered cordless, I can certainly see where that might be a good feature, but all my cordless kit is DeWalt and they don't make one. Also of course being without a cord is useless if you have a great big dust collection hose attached!
One recommendation I do have though, having two ROS with different grit papers on them can really help if you are doing a lot of sanding, particularly on larger pieces. It also saves on wear and tear taking the discs off. If I had no ROS at all I would quite possibly buy two at $50 rather than one for $100.