I had to google that one, never heard of a thermohydrometer before.
Apparently the only real difference is that it has a built in thermometer, which would make correcting for temperature a bit easier. Then again, if you already have an accurate thermometer, it seems like a waste of $8 to me. Ultimately it's up to you, but I wouldn't pay extra, especially not if I had a bad track record of breaking hydrometers.
Umm... not that I've broken many hydrometers or anything. I just... umm... have this friend...
I had one of those thermohydrometers, It didn't seem very accurate to me. The temp never chnaged and always said 70F no matter what (ice water, etc). Also, my gravity readings were always off from target. This, of course, could have been true but since the thermometer part didn't work, I didn;t trust it. I'm sure I just had a defective one though.
I have one and like it a lot. You do have to wait 10 min or so to let the temperature in the sample equalize with the thermometer itself, since there's a decent amount of mass there.