26 for retro and 29 on my modern.
I find I can point my 29er at a lot more and be happy it'll make it over/ through. It makes downhilling crazy fast at times. The fact it has a lot more travel than my two retro full sussers does assists this though.
The 29ers take longer to accelerate but the pay off is the fact once you get going you can get up to high speed and keep it.
I find my 26er's more nimble though.
I haven't tried a 27.5 for a full ride but had a go on a mates and to me the difference to a 26er was negligible.