I have both and like them both very much. The Happy Rabbit is a little bigger and rechargeable. Also the shaft and the ears have separate controls so you can really find what works for you. The Splash one is smaller and shorter, two vibrators as well and in no way lacking strength. Battery operated but it doesn't drain them like a lot of toys do. I've had the same batteries in it since I got it and it has seen quite a bit of use already.
If you enjoy the slimline one now you might not want to get the G-spot version of the Happy Rabbit as it isn't the smallest. Realistic or Natural is your safest option I think.