I've had Grand Slam Tennis and Virtua Tennis for one day and played them about 3 hours each. I've played tennis for years. I'm using Motion Plus.
Here's a comparison.
Tournament mode: Grand Slam wins by a kilometer --- In Grand Slam Tennis you play the Grand Slam tour and do some exhibitions. No stupid "shopping races" like in Virtua Tennis. This part of Virtua Tennis is highly annoying. I don't want to run around a court on a shopping spree and jump through arbitrary hoops in order to play in a tournament. This part of Virtua was so annoying it took Virtua from 5 stars to 4 stars for me. Grand Slam exhibitions / mini games are fun while Virtua is annoying. On the other hand, you can just press "play" and choose tournament in Virtua but the experience is not as full as Grand Slam.
Graphics: Virtua Tennis wins by a meter. The crowd is better and players movement's are much more realistic to the player's style. Grand Slam's graphics are more cartoonish but with the announcers and the angles and replays it is more like participating in a match while you watch it on television... the presentation is better but does not make up for the better Virtua movement and detail.
Speed of play: Virtua Tennis by a kilometer. Your character moves at a more realistic pace and the match is just a more entertaining, fast action pace.
Controls: Grand Slam by a centimeter: Grand Slam's controls are easier to master. Virtua's controls have less tolerance for error. You MUST complete the training coach with Virtua to master any advanced shots, even then the drop shot leaves me wanting to smash the controller! Grand Slam's topspin, backspin, and fade are more true to real tennis, while Virtua's serve, lob, and dropshot are much more true. Grand Slam allows you to "cheat" somewhat by using the nunchuck to help with your crosscourt winner, while Virtua requires you to nail it perfectly. While I appreciate Virtua's effort, sometimes my intentions do not translate, but I suppose that's true in real tennis too.
Overall experience: A tie. Really. I appreciate Virtua's pace and effort at control realism, but Grand Slam gets the top spin and back spin nearly perfect... and that's a huge part of a real tennis player's game. Grand slam lets you cheat using the A button for a lob and B for a drop shot, and the nunchuck for crosscourt slams. Virtua has great realistic smash and lob controls, but the drop shot is just too hard and overambitious. The tennis stars in Virtua are more current and broader. Grand Slam gives you classic stars and current stars. Virtua's graphics are killer but the career mode is like throwing Jar Jar Binks into Star Wars... annoying and distracting. I want to play tennis, not the Sims! They both have room for improvement, but for a quality virtual tennis experience both beat the "plug and play" Wii Sports. Both require a bit of patience to learn. I'll come back to both for different reasons. 4/5 for Grand Slam. Note: I originally gave Virtua 4/5 stars. After playing it more see my warning below! And I now have to say if you are just going to by one, buy Grand Slam.
Edit after playing both for about 1 week: I can't win one game playing Virtua with a nunchuck. It is meant to only be played with the Remote. If you want to play a souped up, fast paced version of Wii Sports Tennis, this is for you. If you want to play a game with the nunchuck that requires thought, anticipation, and Strategy, play Grand Slam Tennis. If I could re-rate Virtua tennis after one week. I'd give it a 2.5 or 3 stars. If I could re-rate Grand Slam after one week, it would get a 5 star rating.