Go to the LDS and rent a few different DPVs, but if your looking for an opinon here's mine :
Get a used Mako/Aeris. I currently have a 1987 Dacor Seasprint same as the Apollo AV-1. I love it. I bought it off Craigslist for $50., and bought a new battery for $60. It had a cracked fan shroud. But when I went to price out the the shroud they wanted $190. I ended up doing a little research and repaired it myself for $10., not the prettiest repair but it works fine. I've had to replace a few other parts that I ordered from directly from Apollo and they were not cheap. What I'm getting at is you can if you find an Dacor / Apollo AV-1 (i'm currently trying to find another one to store for parts) , prepare to pony up if you need parts. You can find some of the parts locally, but not all , for about 1/3 of what it will cost from the "retail guys", but it will take time and an a lot of research. The Mako parts on the other hand, seem readly available and for a reasonable price, for example one prop blade for the AV-1 is $30 on ebay and a complete prop assembly is $520. For the Mako you can get the entire prop assembly kit for $67. (That includes 3 prop blades and parts to build a complete assembly) Don't take my word for just google it and see. The Dacor / Apollo is a great DPV but I'm also on the lookout for a Mako. I'm not going to selling the Dacor when I find a Mako. I do a lot of beach diving and the Dacor suits me fine. It's built pretty solid and I've towed two other divers, at the same time, in full scuba gear, in some pretty strong current. I'm sure the Mako will do the same. My only complaint is having to carry it around. It's about 45 lbs out of the water on top of your dive gear. In the water it is about 2lbs negative but you still know it is there because it is bulky.
If you dive alone, or with someone else that has a scooter, the Bladefish is a good option.
One of the guys I dive with has a Bladefish 5000. He just straps it to his chest on the BC and doesn't know its there until he is ready to use it. But I have seen the Bladefish struggle a little, in a strong current but all he does it kick a little to help it out.
I don't have much experience with the Seadoo but I just look to see what is for sale and there are a lot of Seadoos for sale and cheap. That only tells me that nobody wants them for one reason or another. From your prior post you know how difficult it is to even find a used Mako in any condition for sale.
It really boils down to what you think you need. I like the Mako/Dacor/Apollo because I know they are strong enough to get me and at least another diver out of trouble, if need be. I would like to have one of the fancy tech scooter but it would be overkill for the diving I do.
If your in the Ft Lauderdale area and you see someone with something that looks like Franken-scooter, stop me and I'll be happy to let you take it for a spin.
There is a used Gavin currently for sale on craigslist for $1200. Ft Laud area. A little more than what you want to spend but you never know what he may take.
Good Luck