I've been eyeballing teardrops and other small trailer options for years. For some reason, I had always considered a cargo trailer to be too heavy. Then I realized they're only margainlly heavier than most 5x8 tears. A Carry-On 5x8 weighs 890-lbs. I could even get a 5x10 and gain a side door and two more feet of floor space at 930-lbs. I cannot go too heavy because of my tow rig:

With a cargo trailer purchase being fairly easy and convenient, I may actually make a move this year!

Enough blabbing! I came to ask a question or two:
1) I've learned a lot about insulating the interior by reading here. It's a tad scary to notice these things are mostly body frames bolted to a chassis with aluminum rivetted to the outside and 1/8 wood screwed to the inside.

2) Would getting a 5x10 over a 5x8 be a bad idea? I plan on installing electric brakes on whatever I get. Other than stopping power, is there something I may be overlooking about the extra two feet and 40-lbs?
3) How are most of you addressing air conditioning? Are you using roof mounted units? Or are you cutting a hole in the wall an using a less expensive unit? I've seen a few slick set ups. But I'm sure I haven't seen them all.
In case it matters, I'm looking to get a swing-open rear door instead of a ramp. The 5x10 is attractive because of the side door. I'd likely convert that door to something more conventional with a window or porthole and a door knob. If I stick with a 5x8, then I'll likely cut a hole in the rear door and install a smaller "RV door" for convenience. I appreciate anything thoughts you might have.
Thanks!
Scott