Given a choice between a larger pool and more decking, I'd have to recommend decking is where I'd spend my money.
Other considerations I wish I'd known before building our pool:
1. I'd have an engineering company out to recommend/design any necessary drainage systems. If your yard's on a slope, talk to them about a retaining wall system. (We love the looks of our boulder retaining wall, but an engineer I had out yesterday on a different issue looked at it and said we should have called him first, as, in his estimation, a boulder retaining wall is about the worst in terms of structural integrity).
2. If you want your pool to be as maintenance free as possible, consider an automation system like Pentair's Easytouch/Intellitouch...in addition to the SWG already recommended.
3. Consider a variable speed pump....made even better when tied into an automation system.
4. Don't forget outlets to plug various things like robot cleaners into.
5. Have your builder use as many unions as possible at the equipment pad....makes repairs/modifications/improvements much, much easier later on.
6. LED lights for the pool....we love our Intellibrite lights.
7. If it's in the budget, consider an automatic cover for your pool. (I wish I'd known how many leaves, grass clippings, pollen, seed pods, etc. would blow into our pool!).
8. We chose a sand filter system for our pool because of cost. I wish we'd spent just a bit more and gone with a cartridge filter, as the water would be clearer at night.
9. Lighting at the equipment pad will make nighttime maintenance much easier.
10. Consider long-term things when choosing your equipment. For example, we had our equipment pad poured large enough to add a heat pump later on if we decide we need one. It was for this reason we had our electrician install a 100-amp sub panel at the equipment pad.
11. We didn't know this when our pool was built...but equipment manufacturers will extend their warranty if multiple pieces come from them. For Pentair, for example, if three or more items (e.g. pump, filter, lights) come from them, the warranty on all three goes from one year to three.
12. Don't be too tempted by the low bidder. ( I won't say that's what we did....but I WILL say our pool took over 5 months to complete...LOL).
13. Interview multiple builders. In our case, the builders all had different designs. After evaluating all of them, we chose the one we liked best and had the others rebid.
That's all that come to mind right now....