Most strips are seem to be 30 per meter, the 60 per meter are new to adafruit (this week), and they discussed them on the latest ask an engineer (last weekend). The 60 per are pretty neat as they have the chip inside the LED to save space on the strip, to put more LEDs! I think they have some limitation, but I cant remember what it is (like auto latching).
If the best price is the most important thing, then you probably wont ever want to buy something from adafruit, stick to ebay and amazon. Adafruit does do a lot of educational stuff, so its nice to support them, but you will pay a little bit more.