I realize now I misread your post. I was thinking F20, not E20. I've never flown an E20, but I see that it's a 35 N-s engine while the E9 is 28.5 N-s. All other things being equal, the E20 would take a slightly longer delay than an E9.
When you say the E9-6 flight was a cliff-hanger, do you mean it had arced over and was coming down fast when the chute finally popped? That would suggest that the 6 second delay was long for the E9, but with higher thrust the E20 might well need a 7 second delay. You'd need to run something like Rocksim to be sure. My best point of reference in personal experience is the Aerotech Arreaux which is silghtly heavier than the Tomahawk (12oz vs 9 oz) and just a bit longer. For E engines, it uses the E15-7 and E30-7, both of which are 40 N-s engines. This delay seems to be dead on for the Arreaux, so I'd go with the E20-7 in the Tomahawk.
As far as wind goes, the faster the rocket is going when it leaves the rod, the better and the E20 has twice the peak thrust of the E9, so it's a much better choice for windy conditions.