I've played games from all of those series, and never really found any major difference. Hell, when they make game likes Streetfighter X Tekken, its pretty obvious the gameplay is quite similar.
Not to say that I hate any of those series. I'm perfectly happy with all of them, especially StreetFighter.
Actually, many Tekken fans dislike Street Fighter x Tekken, because it plays nothing like the Tekken series. The only core gameplay mechanic from Tekken that made it into SFxT is juggles. Other than that, SFxT plays too similar to Super Street Fighter IV, except a little more agressive/fast paced. Street Fighter, and Tekken crossing over is exciting, because those are two fighting games that play completely different from each other, so everyone was hyped to see how the game would play. Capcom's version played too much like SSFIV, and Namco's version would most likely play like Tekken games (with a few SF elements added to it, I believe Harada said he was going to add an ultra meter to the SF characters).