1) If I were shopping for a tube today, I would be looking for stability, overall weight and portability. I want a stable tube that will sit higher out of the water but not so high where windy days will cause some issues. Knowing how far I tend to walk from the car to the edge of the water (and back) also has me thinking of how heavy tubes can be. Lastly, even though I have an SUV, I like to know I can pack it away to a smaller size just in case I need to transport more than a tube.
A lot of people also like to look at how much storage space a tube has, but at this time...we can get very creative with pvc and plastics to build what will fit/we need.
2) When I purchased my first tube, I did a lot of research. However, I never took into account of overall weight and other small issues that I mentioned in the previous answer. It was an ODC 420. I loved the tube, but man, it was heavy and it didn't pack away so nice due to the heavy construction and super thick seats. Mind you, I didn't have an SUV at the time and was driving a two door coupe. I made it work, but the transport to/from the car and the pack up left a lot to be desired.
Keeping that in mind, my second tube was super light weight and super compact. It was a Caddis pro 2000. It's a super low riding V shaped tube and was light as a feather. However, with the weight reduced, it also meant I wasn't going to sit as high. I was often sitting in an inch or two of water. It says a lot too since I'm a light weight (130lbs). Also, it didn't have much reinforcement so I had to be more careful in fear of popping one of the chambers
I loved how wind never really gave me an issue, but due to sitting lower in the water, it caused me to use more energy to move around. Also, the storage tilted a little inwards getting in the way of your elbow. I did appreciate having a cross bar to reinforce the tube/netting tray though.
3) I currently ride a cumberland. Its a great tube and meets my needs. It's lighter than my ODC and sits higher than my Caddis Pro. Ample storage on the side pockets that dont get in my way. Also it is reinforced in the right places so I don't need to be super weary of sharp objects. Things happen and it can still rupture (search SP Dan's post where a Sculpin almost sent him to davey's locker).
4) If you are looking for a tube, ask to try some different ones out. U shape, V shape and the super fancy maxxons. Find one that will fit your fishing style the best and will see the most use. If you are hiking to honey holes it would make little sense to pack a heavy tube and it makes little sense to use an ultra light in a super hazardous area. Don't be afraid to look in the secondary market!! THere are lot of used tubes out there that work perfectly fine at a fraction of a cost of a new set up. Sellers often have packages including fins/racks and pumps. Look around before buying, deals pop up often. Lastly, remember tubes can be modular. As long as you are comfortable with PVC/Glue/screws, you can make anything work to fit on a tube.