Played both extensively, but now exclusively play Full Ace. For me it's undoubtedly the best tennis game ever made. Compared to any other tennis game, it has the best gameplay and control mechanics - steep learning curve, but unbelievably realistic with how you time and aim your shots. Ball can go anywhere, miss hits, realistic errors etc. Great how you can customize so much, cosmetically (eg. camera, screen indicators, ball visuals), and more importantly, gameplay/control/ai difficulty... I put everything on the most difficult, most sim-like settings (ie. level 24), except for timing (level 10/24), which makes the game a real joy to play.
Mods are fantastic, generally coming out at main tournaments. TE does have better support which i find unusual since I consider FA to be the far superior tennis simulation.
Finally, a recent update, has dramatically improved volleys to an amazing level of realism.. Feels so good. Its probably one of the hardest things to get right in a tennis game. Oh yes, this game also has an awesome tutorial mode, describing by example, and then getting you to actually perfectly perform the skill before moving on.
Cannot really ask for more, considering this is a one man team. The only thing Id love to see added to the game is player diving or stretching for an almost unreachable ball, and would absolutely love 'vibration/controller feedback' support - would add so much in terms of immersion. The developer has indicated (via the steam discussion) that this will occur in a future update. Can't wait.
Edit: There really needs to be a
Full Ace Tennis Simulator thread opened up... May do it myself and post some helpful video, of my settings, which i consider pretty good

camera wise, and realistic wise, considering i have many of the default screen indicators hidden for realism. Here is an old video i posted ages ago which shows how i usually play (these days ive slightly changed my setup - which ill post in another thread):