A99 V GPE27 5/8 WAVE

A99 would be my choice but then i haven't used either for many many years.

If I was going to buy one today it would be the GPE 27 5/8. That is purely on the basis that I have used one in the past, it worked it well, plus the theory that more metal in the air is often a good thing... The 5/8 is longer/taller and has more girth  than the A99 which only has a single wire radiating element inside its fibreglass shell.

If I was on a hill or by the coast I might go for the A99 as many people rave about its weather resistance.

I'm not a betting man, but if I was I'd put a tenner down that if you bought both and set them up side by side at the same height you'd struggle to notice a difference in performance.

Thanks for your can o' worms question, one like that is always a good convo starter  If you are looking to buy one, do let know which you choose and how you get on with it.