My general feeling on something like this goes something like this.
1 - buy the biggest box you have room for. There is nothing worse than running out of breaker slots. You already discovered that on your main.
2 - Anything over 1 (real) hp should be wired for 220v and needs a 220 breaker. The good news is you can use smaller wire in your shop and loose less to heating the wire. The bad news is they take up two breaker slots. They do have dual 220's and 220's with two 110's on the side though so you don't really loose that much.
3 - Picking
http://www.electricsupplyonline.com/wire-romex.php as a random place to check prices, 2/3 romex goes for 1.79/ft vs 0.94/ft for 6/3 romex. I would expect other places to charge using a similar ratio.
4 - There is a fine line between stingy and frugal. I try to be frugal. On the other hand I would rather buy a little more than I need than a little less than I need.
5 - A decent dust collector suitable for running a small shop like
http://www.grizzly.com/products/item.cfm?ItemNumber=G8027&site=grizzly will draw 14amps of 110v, 7amps of 220v. 500 cfm is the right size for one large tool or several small tools.
6 - you said you have already run a circuit from the main panel to the shop for your air compressor. This suggests that it is not that hard a run.
So, my initial gut reaction based entirely on how the light reflects off the throttle lever on my kubota is ... I would seriously think about dropping two subs. A 60 amp sub would be overkill for the living area, but a 60 amp sub would be sufficient for a shop allowing for easy expansion. Materials would probably cost about the same as running a 100 amp sub to service both. Depending on how you would have done the appliance outlets in shop and living quarters, labor might even be about the same.
Price it out different ways and see what makes the most sense for you.
