If you can get the right weber adaptor then that's good, I could never get one that went to my manifold. I ended up making one.

I like the weber it goes better and it lots more fun. You'll probably find it to be less reliable than the oem carb. Maybe with enough tuning and messing around it can be as reliable as that stock carb.

Reliable?
It's hardly ever done anything so bad I couldn't fix it or left me stranded.
Get a very good filter for the fuel, webers seem to get plugged up with very tiny stuff that the oem carb didn't mind.

I love my weber and A15. With the right setup I think it's lots of fun.

there are alot of variables, between a14 and a15, there are an assortment of heads, with different shaped ports and combustion chambers.

I think it's probably reasonable that the a14 might have a Hp spike above where the a15 levels off, the a15 will have more torque up to where the torque / HP crosssover point where the A14 will make a little more HP, but the A15 having made more torque off the line would be faster.

someone would have to have an a14 block and an a15 block and swap the head and intake and exhaust to really answer which is better

some people really like the a12 it's got a really short stroke and revs even higher and makes more hp than the a15 in some cases