The ignored factor --> contest risk. A vertical spread that matches the underlying in terms of delta will be a LOT more costly in terms of commissions and bid/ask spread. Plus delta will go and change on you as time goes by, vols change and underlying moves.
The way I saw it was that an initial position is near-linear... And gamma would be low. Delta would increase over time due to decay... So you're getting "more shares" as time goes by. But I didn't consider b/a. However, the ATM vertical is much cheaper than the underlying for a spread with equivalent delta