It's sort-of a "pick your poison" kind of deal. The larger the antenna you purchase, the more gain they generally have. However, at some point, it gets ridiculous to carry around. Most people end up purchasing one of the Comet, Diamond, or Maldol antennas. They're all really made by the same company. Find one that is for the bands you want to use and is a length that you can deal with. It's helpful if you can look at the antenna in person to inspect how durable it is. I went through several Comet HT224 antennas because the base separated from the rest of the antenna. Fortunately they replaced it under warranty and the antenna is now redisgned so that won't happen. Check the specs to compare the advertised gain and bandwidth. The Comet SMA-24 advertises gain specs of 2.15dBi on 2m and 3.5dBi on 70cm, but it's also almost 16" long. Also check out the Jetstream Antennas. They're supposedly the same antennas as the Comet's and Diamonds and made by the same company, but manufactured in China instead of Japan.
If you can go see the antennas in person, see if you can set up a test with the guys in the store. Leave them with a credit card and ID and then take a few antennas about a mile away and interchange them. Watch your S meter and also get the other station's feedback.
Let us know what you decide on and what you think about it after you've had it for a while.