Home pools are not usually large enough to swim in. We decided to get an in ground spa since we will mostly just enjoy the water and jets anyways. We can use the neighborhood pool for swiming. Just about anything you add to your home can be done less expensive after closing. The question is do you want to put up with the noise, dirt and annoyance. Building a pool after closing will surely destroy a lot of landscaping that will need to be repaired as well. I believe most people get pools for the look and feel of the home rather than usefulness for swimming. The comparison of cement and fiberglass is laid out well here:
Fiberglass vs Vinyl Pools, Fiberglass compared to Vinyl Pools