If you want the Cadillac of scopes, you are looking for the Pico. It's a great lab scope, if a bit intimidating for a new user. It has a couple of flaws in it (search for "pico gap" on iATN or autonerdz), but nothing that knowledge and a couple simple workarounds can't fix. The biggest downside in my opinion is the time it takes to set it up, and fire up the computer if you don't have it running already. The Pico also has tons of support behind it at the Autonerdz forums and at iATN. You can actually play with the Pico software, as it's a free download from the Pico website. A similar setup is the Escope Pro, but not as many people use that setup.
If you want a simple scope to set up, and the advantage of connector pinouts and example waveforms in your hand, you are looking for the Vantage Pro/Modis. The biggest advantages these offer is the connector pinouts and locations (saving time looking them up), the quick bootup time (10-15 seconds from power on to looking at signals), and the auto setup for unknown signals. I don't know what kind of user you are, but one thing to consider is that while the Modis has four channels, you can't use the scope and the scan tool at the same time. Obviously the Vantage Pro doesn't have the same problem.
I wouldn't bother with even looking at the add-on scope for the Genesis. It's a pretty piss poor scope, and not very user friendly even for people who have used it for a while. We've got one at work, and if I had a choice between using that for a scope and working without a scope, I'd go without. It's honestly that bad.
The UEI scope is comparable to the old Fluke 96/97/98 which are pretty outdated by todays standards, and it only has an 800 point record length, which is about two screens worth of info on the Vantage Pro/Modis. Not exactly a great selling point for a $1300 scanner/scope.
Personally, I've got a Vantage Pro and I'm very happy with my purchase, and I've certainly made my money back with it. I'd love to test drive a Pico at least once with actual data, but I'm not going to go out of my way to take one for a test drive.
The thing to keep in mind is that the best scope is the one that you use. You can have the fanciest scope in the world, but it's pretty useless if you don't actually use it. That's part of the reason I settled on the Vantage Pro, and I still use my original Vantage for most things. For my type of use, they are just easier to set up and use. You also need to use them all the time, not just when things are broken. Most of what you are looking for are subtle differences between good and bad, and it takes a fair amount of practice to be able to spot those differences.