I think it's a matter of taste and rifle.
Tango Down makes a rail mounted unit that folds flush against the sides of the rails. Pros: Folds flush against the rails. Cons: Lacks adjustment.
GripPod is a vertical grip that contains a spring loaded bipod. Pros: Convenience, cool factor and freeing up rail space. Cons: Lack of adjustability, the shooting position at which the bipod puts you when prone. Beware of imitators - they are cheap, crappy and will likely break down quickly. Real GripPods are about $100.00
Harris is really the industry standard. It's likely used by more shooters than any other bipod. Rail mounts can be had for $15.00 to $100.00 If you have the budget, a Harris bipod in a LaRue rail mount would be kick ass....Pros: Rock solid, adjustable shooting heights, myriad of options, (cant, rotation, leg lengths , foot options, etc). Cons: Bulky, slightly complex and unless mounted on a QR, nit easy to mount/dismount quickly, (much like my old girlfriend)...
The one thing to consider is what kind of rifle you're putting it on. Generally, putting a bipod on a fighting carbine is not a popular thing to do - "elitists" have a field day with people putting them on M4's for example. I say: Screw them - if a Harris makes you happy and encourages you to shoot more, then go for it. I don't have one on my M4, but have one on my SOCOM-16/Troy MCS and love it....