To the OP -- according to your profile, you'd be using the scooter in open water. If that's the case, size and weight DO matter, because space on boats is limited, and boat crews aren't always happy to drag 70 pound scooters back on board for you. Going through surf with a long, heavy scooter can be a challenge, if you are shore diving.
In addition, scooter size and shape has a impact on what the scooter will or can do. Short, light scooters like Sierras are very nimble, and are great for doing loops and rolls and general scooter shenanigans. Longer, more massive scooters are more stable and better for long straight running, like power tunnel in caves, or using a scooter from shore to reach what are normally boat dive sites.
Battery technology is important, too. Lead-acid batteries aren't very common in scooters these days, because the batteries are so massive and heavy to deliver the needed power, but such batteries tolerate deep discharge repeatedly much better than NiMH or LiIon ones do. LiIon packs much more punch for the same battery size and weight, but scooter-sized LiIon batteries are VERY expensive, and cannot fly (over the lithium limit).
Scooter speed control can vary by model. I like my Sierra, which has five speeds you can select with the trigger, so you can change what you are doing very quickly. Other scooters are set up more for you to select a speed and stay with it, which is ideal if you are running a team together over a long distance.
If you are looking for an open water scooter that is easy to get on and off a boat and will make it easy to play, I'd recommend a used Sierra. It's hard to beat the value.
Beano has a good point, too -- remember that it's very easy to get yourself into a lot of trouble with a scooter. Running rule of thirds on the battery is an awfully good idea, so that your buddy has enough juice to tow you home if the scooter fails. You can get a long ways from home very quickly on one of those things!
