I personally tend to favor the 7-die sets when I buy dice, but I also already have a number of sets. Are you primarily playing or DMing?
If playing, I recommend a 10-die set if you can get it (1 d20, 1 d12, 2 d10 (d%), 1 d8, 4 d6, 1 d4). That's plenty to keep a character busy until 6th level or so. A 7-die set will last a good while, though -- you'll just have to re-roll the d6 a lot.
If DMing, I find that 4 sets of dice come in handy -- I find that having at least 4 d20s, 2 d12s, 2 d10s, 4 d8s, 4-8 d6s, and 4 d4s covers most situations until you get up into the high levels (d10s and d12s don't get much use, so you can get by with fewer).
One useful consideration if you DM ... don't get all your dice in the same color if you get multiple sets. I find that having multiple d20s in different colors is useful for rolling multiple attacks simultaneously. If you get damage dice (d4-d12) that match the d20s, you can roll multiple attacks and damage simultaneously, keep the attacks/damage separate, and save lots of time. So, if you were to buy 4 7-dice sets, get each set in a different color scheme, but make sure that within the set they are all the same. Make sense?
That's the real benefit of Chessex matched-color sets over WotC (or other) mixed color sets IMO -- having matched d20 and damage dice really makes you DMing combat job quicker.