I thought all 737s after the 200 series - which had little Pratt and Whitney engines - have CFM-56 varients.
The 737-300/400/500 have very flat bases and the 600/700/800/900 are not as flat but they are definitely not round.
But there is no choice of engine on the 737 as far as I know.
Airbuses have CFM-56 with round nacelles (because they are higher off the ground than 737) - or IAE whatever engines.