As an avid tennis fan/player, I am happy to succumb to debating unanswerable questions about tennis players. These useless debates requires statistics, and this nice representation of the data regarding the Grand Slam triumphs of Sampras and Federer is helpful now that they are tied.
Federer's ascent has been shorter and steeper, with him routinely making the semifinals; he's won three-in-a-row twice. Sampras has many weak showings, particularly at the French open; he's won three-in-a-row once. As one can see in a chart from another comparison, this trend extends to all titles.
What's also interesting is that Sampras and Federer seem to be leveling off at the same age. Will Federer win 2-3 more slams as Sampras did in his tennis dotage, or will he fade as quickly as he turned on? My guess is that he will fade more quickly than Sampras.
Around this time in his career, one could see Sampras' power game taking focus. He overwhelmed Agassi at the 1999 Wimbledon final, missed the US Open due to a bad back, then again rolled Agassi to take the year-end championships. While Sampras always had a big serve, forehand and overhead, he perfected his half-volleys and passing shots late in his career to dictate play and avoid long rallies. His power game carried him to the 2000 Wimbledon and 2002 US Open titles despite slumping badly overall.
In comparison, Federer's all-court style, while magnificent to watch, is perhaps less kind to old age. It requires superb footwork and stamina to get into position and hit the ball cleanly. It allowed him to prosper on all surfaces, but may relegate him to also-ran status as his feet slow down. Does he have the power game to keep the points short and win Grand Slams in 2010 and beyond? He displayed excellent serve-and-volley skills against Sampras in their 2001 Wimbledon epic, but I fear these have fallen into desuetude as the game has shifted towards baseline play.
One interesting alternative is to use his fluency in tennis shotmaking to befuddle opponents, much as Fabrice Santoro did well into his 30s. We got a taste of it in the French Open final, where he used the drop shot and lob very effectively against Soderling. This strategy not only kept Soderling honest, but delivered many outright winners without requiring power or speed. Supplemented by his hot serving, he completely dominated Soderling from start to finish.
If Federer has an ambition to regularly contend for Grand Slam titles in the next several years, he would do well to keep expanding his game. If he can also keep getting cheap points on his serve, he will very likely prove me wrong.