I went from 10D to 30D to 50D and now 7D.
Compared to 30D, a 50D will give you approx. one more usable stop high ISO. Compared to 30D, 60D would give you about approx. two stops.
Both 60D and 50D would give you arguably better AF... The 30D has AF similar to 5D/5DII (except it tracks movement better than the FF cameras). It's got one cross-type point, at the center. The rest are single axis AF points. In fact, today all the Rebel/xxxD series use the same or similar to 30D/5D/5DII... except for the T4i, which appears to inherit its AF from 60D. 40D, 50D and 60D have all 9 cross-type/dual axis points, though the center one is still further enhanced.
30D is 8MP, 12 bit. 50D is 15MP, 14 bit and 60D is 18MP, 14 bit. Both 50D and 60D use the Digic 4 processor.
The metering system in 50D is similar to the 35 zone system in 20, 30, 40D, 5D, 5DII, as well as some other models of the same generation. 60D uses the newer 63 zone metering system that's found on 7D, 5DIII, and most (maybe all) of the current Rebel/xxxD models. In use, I don't see a whole lot of difference. I think the newer metering system handles backlighting and some other tricky situations a little better.
50D and 60D both have self-cleaning sensor. I have to do manual sensor cleaning far less often now, than I did with 30D and earlier.
60D can shoot video and has that neat articulated LCD screen. 50D can't shoot video and has a fixed LCD screen. Both camera's LCD screens are considerably bigger and brighter than the 30D's.
50D has MFA or Micro Focus Adjust, to allow fine tuning up to 20 lenses' focus to match the camera better, if you wish. It is not limited to macro lenses only... it can be used with any and all lenses, AFAIK. 30D and 60D both don't have MFA. (5DII and 7D have MFA. And 5DIII has a newer, fancier version of it.)
50D has a control layout similar to 30D, 40D, 5D. 60D is sort of a hybrid, with some controls that are a little more "Rebel-like". It's not bad, just different. For example, it's got a multi-directional button instead of a joystick. On a bright note, that's place so it can be reached with one's thumb, whether using the camera in landscape or in vertical/portrait orientation, even with a vertical grip on the 60D.
Both 50D and 60D allow interchangeable focus screens of a few types. 30D's isn't interchangeable.
60D is more plastic, 50D is more metal. Despite this, 60D feels substantial in hand and seems equally durable (plastics... or polycarbonates if you prefer... aren't necessarily a bad thing).
60D has a latch on the mode dial, that I wish were on every Canon DSLR with the easily bumped and accidently moved mode dial on the lefthand shoulder of the camara. It can be retrofitted to 7D and 5DII (at additional cost0, but not to 50D or other models.
50D uses the same vertical grip, batteries and memory cards as 20/30/40D. 60D uses a different grip, LP-E6 batteries and SD memory (which might be why it has a slower top frame rate).
Really, I think either camera would be a very nice upgrade to 30D. Not that 30D is a bad camera at all. It's still quite usable. But the higher resolution and the higher bit rate allow for more cropping or enlargement... and there are a lot of other incremental improvements, as you can see.

