It's hard to say w/o knowing how much of a bonus Spanish (and other infantry) is getting and the formula used to calculate the results. Example:
Let's suppose a regular roll has a success chance of 1/6. 8 Gallic infantry units will roll 16 dice, so it's 16/6 (1/6 * 16, etc). If Spanish infantry has a success chance of 2/6, it's also 16/6 for 8 Spanish infantry units (2/6 * 8). However, if Spanish infantry bonus adds up to 3/6 then it's cumulative success is 24/6, which is way better than Gallic infantry. General's rating effects the dice rolls as well and we have no idea how. So, unless James spills his secrets, it's a guessing game.
My own infantry preference for city and open field battles (provided I have no particular unit shortage) is to mix of 3-5 Spaniards and Gauls, backed up by 1-2 African/Italian infantry to absorb losses. You need Gauls to create more chances, while you need Spaniards to capitalize on a few that you've got.
If I wanted to give a witty, but less helpful answer, I would have said Elite Macedons.