Im sorry but no. You pay 1,000,000 bells and only choose one(unfortunately)

No u can't which sucks,after u pay 1,000,000 bells pelly asks which one u want and is permanent and u cant change it so choose wisely

No, you can't get both. You will only get one chance to choose and it's permanent.

The windmill boosts your town rating and helps you achieve a perfect town.

The lighthouse makes it easier to get rarer fish by attacting them.

My personal preference is the lighthouse because you can reach perfect town status without a windmill, and once you have reached it and gotten your golden watering can and whatnot, the windmill becomes useless, whereas the lighthouse will always increase rarer fish spawn.

No, but I like the lighthouse better.

Lighthouse increases your chance of catching fish by 30% and windmill makes your town better by 20 u pick like rarer fish or more better town,its your opinion but i like the lighthouse because selling money to nook is better then just getting a gold can

I haven't done it myself, but I've been told that if you are unhappy with your selection or just want a change, you can donate an additional 1,000,000 bells and Pelly will give you the option of changing your lighthouse/windmill to the other option.

As to what the lighthouse and windmill do for you, I've head that they are only decorative and do not increase your chance of fish or make your town better. I guess it depends on what blogs you read and who you believe. I have friends who have the lighthouse and they say they have not noticed a difference in the fish they catch.