A14, or A15? Decisions, decisions.

With all things being equal, the extra 5mm of stroke in the A15 "should" result in more torque. This in turn results in greater flexibility, or the ability to hang onto whatever gear that you are in when under load, such as when hillclimbing, or towing / carying a load.
I run a stock 1200 diff center in my 1000 sedan with the mildly modified 1200GX, 1 to 1 five speed & 13" wheels. With this combo i was forever looking for another gear.
A 1500 would benefit more from an overdrive box & / or a higher final drive ratio as it would have the torque to pull it better.
At my age, torque is regarded as a good thing as my enfeebled arm doesn't flick the gearlever like it once did.

The A14 is every bit as good an engine & will have the ability to rev a little higher, although it is all a bit academic as any of the A series can rev quite well enough to satisfy any road car need.
If you have an A14, then use it, & you will be more than happy with it as they are a great engine, & hey, all you are doing is upgrading to the stock "Bakkie" specs, so even the factory says this is good.

The engine thats going into my KB10 will be a little over 1400cc,[A12] but i need to come up with a diff center that bolts into my stock rear axle that has a ratio a bit taller than 3:9 to 1. Somehow, i don't think one exists.
Chris