Round vs oval? Hm....if you want to swim laps you need at least 33 feet--and that's barely enough. A 33' round will have 25,000 gallons of water. An 18x33 has around 14,000 gallons. (assuming 4'water depth) That means the round will need more of every chemical, and takes longer to "turn over" the water everyday--or requires a bigger, more-expensive-to-run pump. The middle of a 33' round is 16.5 feet from the side. The middle of an 18x33 is never more than 9 feet. If you are vacuuming manually, that makes a HUGE difference.
All pools must be level and the oval has a smaller area.

A well-built and well-maintained pool will last for many, many years. My parents' 18x33 oval lasted for close to 20 years. Actually, longer. The liner failed and Mom gave the pool to someone who disassembled it and removed it.

Yet again, a pool store guy is trying to talk you into what is best for HIM and HIS store, and not what is best for you.

Carl