A two piece wetsuit will generally have two layers of neoprene over your torso: upper thighs, hips, stomach, and chest area, but only one layer everywhere else. (stating the obvious).
So in general, a 2 piece wetsuit will provide more insulation in crucial areas than a 1 piece wetsuit of the same thickness. A 2 piece will also be more buoyant, and restrict movement a lot more than a 1 piece. You might be trading one aspect of "comfort" for another.
Both types will "leak" water about the same. It really depends more upon how snugly the wetsuit fits to your ankles, neck and wrists. For a 2 piece, the water may take a little longer to get into your stomach area, but that is really more dependent upon the fit than the style.
One advantage of the two piece wetsuits is that you *might* be able to buy separate pieces of different sizes. When I was first renting, I had a hard time finding a jacket that fit, but it was easy to find Farmer John bottoms that fit perfectly.
Other factors to consider: 1) A 7 mil wetsuit can be fairly bulky and heavy for traveling. 2) Many people prefer to dive in a wetsuit that no one else has peed in. Probably more of a mental thing than a real health issue, but who knows. Of course you are diving in fish pee the whole time.
My first reaction to your initial posting was that a 7 Mil wetsuit of either type might be awfully thick for Cabo, even in "winter". If you really need a wetsuit that thick for the relatively warm waters of Cabo, then maybe you should bring your own wetsuit. It might be difficult to find any that thick to rent in Cabo. Check some dive ops.
Also remember that just like when you are snow skiing, you lose the largest percentage of your body heat through your scalp, so for a person that gets cold, a proper hood that fits well would be a really key element.
Diving is just one of many sports where it is very important that you have the "right clothing". If you are not comfortable, you will not enjoy it. If you are not enjoying it, then why are you doing it?
My recommendation would be to buy a good quality exposure suit as soon as you can afford and justify it. Better to rent for a little while rather than sacrificing quality or fit by buying something "cheap". Make sure that you get one that fits really well, and make sure that you have a good quality hood.
Just my 2 millimeters worth,
Wristshot