The Betsons are often less expensive.
If you do go for the Betson style you'll want to swing by Groovy Game Gear to pick up a booster kit.
GroovyGameGear.Com LinkWithout the booster kit, it seems like it takes more spins of the ball to get accross the screen. (hard to explain).
The replacement wheels are metal instead of the flimsy plastic that Betson uses.
Other than the encoder wheel difference, I'd say Betson is the way to go... with the booster kit (opinion based on cost).With the kit, it is truly comparable to the Happs.
-Goz
(I am not affiliated with Groovy Game Gear)