I replaced a Single Rec with a Legacy and have had some extensive tryouts with a JSX. My arguments for your situation:
LEGACY-
Pros: Can be had for very little $$ on eBay, they're an even grand for a new combo. They sound really excellent and they're very fun amps to play.
Cons: They are LOUD!!! Even with the 1/2 power switch The combo a little less so because its only 2x12 but holy crap it'll rattle windows at 3. You'll need an attenuator to actually USE the thing. I run an Airbrake with mine and it's excellent.
JSX
Pros: Very tonally versatile, can get lots of different flavors. Can do everything the JSX does and do it better. The built-in noisegate is cool, too.
Cons The combo version is a little harder to find, and the head does not have a 1/2 power switch. Needs good tubes and great speakers to really shine. The tone is somewhat lackluster in spots compared to the Legacy (the legacy has a much better clean channel). Its a loud amp too but it also needs to be cranked to sound worth a damn i.e. Marshall.