Well, depends on what you are looking for. I think, for "Micro" subs, they all look and have almost the same method of diving:

-Wasaki Mini sub
-Hi-tec Sea Explorer
-Nikko Sea Tiger
-Radio Shack's Mako Sub

These subs mentionned above all have the same system: Two motor pods on either side of the hull, facing downwards at an angle to achieve diving. They are positively buoyant so when not moving, they rise to surface. To turn, one motor stops and the other works. I personnally like the WASAKI sub (very small and cute!) because it has a "Super Dive Button" which allows it to dive at a higher speed than usual. Also the battery can also affect the buoyancy, some batteries are lighter than others so you may encounter some problems while trying to dive.

-Motorworks USS DALLAS / USS CHICAGO
-Nikko Seawolf


These subs have a different diving method, the USS Dallas uses thrusters for all movement, the rudder and dive planes can't move (I believe)

The Seawolf on the other hand uses a propeller located on the under side of the hull to make it dive. As for turning, I'm not sure about it.

They all come in different sizes, I got my Wasaki Sub for 19.00$ at my Radio Shack, very cool!

Hope this helps