Slitherine

What would be a better combo, an infantry or tank unit with a D+6 or one with a D+3 with an A+3?
Thanks for any opinions.

The honest answer is "it depends".
The combat result calculation means that you get a big difference in odds when A and D are within about +/-4 points, but less benefit once it's more than that. So +6D isn't very useful on a Churchill croc since D is usually already so much higher than most attacks that you'd not gain much benefit, while higher attack would be much more usefull with that 140% RoF. However, +6D could make it very useful for wading through close terrain where it would be reasonably resistant to most infantry attacks. Similarly +6D could make a Nashhorn into a very nasty opponent, while +6 A would be a bit of a waste.

I've never got a +9, but I did once have a +8D ranger unit in US Corps - almost indestructible in close terrain though +5D and +3A would have been better, and a strat bomber with +6A was very good in naval scenarios.

SPOILER ALERT:::

Breslau Manstein playthrough, one of my regular infantry just was awarded a +3 Attack 3rd hero to his +6 Def 1st 2 heroes.

(He has and now is even more so a beast)