I don't know if I can answer your specific questions, but I can give you my own experience with 1st gen vs. 2nd gen and you can take what you want from it.
I owned a 1st gen '05 Base (1.8L) for the last 6 years, and I've owned an '09 GT for the last week and a half.
Cargo/hatch area -
The point goes to the 1st gen. Although in the 1st gen you have to deal with two wheel wells and in the 2nd gen you only have to deal with one (not sure how that works but ~shrug~ that's how it is), the 1st gen is still, and by far, the better cargo area if you want to use your Vibe as a 'I like a car, but it can function as a truck when I need it to' vehicle. The 1st gen is solid plastic and truly functions like a truck bed. I hauled everything in that car from a 60 gallon fish tank (with stand) to 20 patio pavers (not light weight!!) and everything just slides right in easy-peasy because of the all-plastic design. My new 2nd gen has rubber runners every few inches. I get the idea behind the design change - the rubber prevents things from sliding around in the hatch - but the problem with that, if you want to use it regularly for hauling stuff, is that: it prevents things from (easily) sliding into the hatch.

It would be very hard, now, to slide a four foot fish tank and four foot stand into my hatch because the rubber would be such a hindrance that basically a second person would need to be inside the car lifting up the back edge of whatever you are pushing in so that you could continue to 'slide' it without it getting stuck on the rubber.
The second issue I have with the new/different cargo area is that in the 1st gen there are eight little half-moon loops that flip out that you can attach bungees to. This new 2nd gen Vibe only has four of those little half-moon things on the bottom of the cargo area. That doesn't allow you to hold anything with bungees unless it's a super thin item. I used to criss cross my bungees in an X to hold things and now that's not an option. If your hatch is completely full it's no biggee, but if you need bungees then it's likely going to be a pain. I haven't hauled anything in this one yet so I can't be positive, but I immediately noticed, and missed, that feature when I noticed I didn't' have those little loops anymore.
The last thing - that goes with what I mentioned above - is that in the 1st gen there are two little cubbies that would hold my bungees when I wasn't using them (and a small lantern). The 2nd gen has these cubbies too, BUT, unlike in the 1st gen where they were solid/sealed little areas, these in the 2nd gen aren't solid on all sides so hypothetically, if you put something in them to store (like bungees), normal vibration from daily driving could shake those items loose and into who knows where those open spaces in the back sides go in to.

The (outside) wheel wells and then out onto the road maybe ?? I have no idea, but it's a poor design change for sure.
As for just what a Vibe can haul, I already mentioned the 60 gallon tank (4 foot) with a wooden stand, and the patio pavers, but I also hauled 16 40lb bags of dirt in the back of that Vibe. It sat VERY low, but it did it.
I took the 1st gen from Michigan to Florida every Thanksgiving through various routes. This was not hauling 1200 pounds so I can't speak to that, but I can tell you that the 1.8L engine did just fine carrying my bags, a cooler, my 100lb dog, and me through the Virginias' mountains, as well as through Kentucky's. Handling was never a problem.
Power -
The point absolutely goes to the 2nd gen. This Vibe has a 2.4L engine and I really didn't think it would make THAT much of a difference but boy was I wrong!! In the 1.8L Base it very obviously was working hard to get up to speed to merge onto the freeway. With the 2.4L most of the time I have to slow down because I don't even notice that I got going SO fast SO quickly. It's night and day, and there's really nothing more to say about it than that.
The body-size stuff I can't really speak on other than to say that the Vibe is surprisingly roomy and I think you'll be okay there. The tilt steering is very helpful if you sit close, as I do, so I'm not imagining, even with the height difference, you'll have a big problem there.
As far as the 2 vehicles you're looking at, I would skip the AWD. I did a LOT of research myself prior to purchasing this GT this time around and the significantly lower gas mileage along with the added cost an AWD can bring if there does end up being a problem just isn't worth it in my book. BTW, my '05 Base had 130,000 miles on it and ran like a charm right up until it collided with that deer. Never had a single problem in 6 years. Not one. So I wouldn't worry too much about mileage or even age of the vehicle. I'd still be driving my '05 if the frame hadn't been bent, and that was the second deer collision in that car and still not a problem one. It purred like a kitten, and I feel like I'm really gonna miss it even though I replaced it with another Vibe.