I had this question a few years back trying to decide to go for a 24 or 27. We went for the 27 and I am so glad we did. You are only talking about 3 feet in the difference, but it sure does make a difference! We were at a neighbours pool which was 24 and I could really see and feel the difference. I say if you have the space..go for it! Of course keep in mind, the bigger the pool, the bigger solar cover, reel, etc. you will need, the more $ they cost.
The kids will enjoy it whether you go for the 27 or the 30'. Just my thoughts! Good Luck on your decision.