Terror from the deep was much better to me than UFO. although largely same (very similar) it does have some new graphics. maps are a bit bigger and nicer. for example passanger boat terror site... with exploding kitchen (i wonder why so many aliens are there) and casino!
in UFO most maps look the same. over and over and over .... i got sikc of all the barns. ok occasionally there are some woods and desert. but mostly it's just same old barns.
and there is always a lot of aliens even if the ship is quite small. which is kind of a bonus
but people who say they weren't blown away with TFTD are right. gameplay is the same. graphics are improved but not as much as one might expect. there are new settings (underwatter - vulcanic, old ship, 2x "normal", gas pipes and underwatter drill facility, new terror sites - 2 ships, tropical islands, port (docking bay), two part missions - colony, alien artefact center, 3 part mission final assault). all in all it brings some new stuff. but aliens are very similar only look a bit different and wear a new name.
Oh strongest weapons no longer has auto fire, which kind of balances it.
well since i played Xcom2 first i was dissapointed with UFO for the similar reason other were disspaointed in Xcom2 :bleh: