The airside hotels have perfectly acceptable 3-4 star quality rooms. But the hotels themselves have limited facilities so you would probably rate them 3 star overall.
There are literally dozens of food and beverage places (and bars) in Singapore airport. No trouble hopping out for a bite to eat at one of those. Or you can go to the Singapore Airlines lounge and eat/drink there.
The airport hotel in T1 also has a pool which is kinda cool and a refreshing way to unwind.
The airside hotel will likely be the most convenient - no immigration or security and easy to get to. Plus, if you wake up at 2am you're still going to be able to get food and beverage in the lounge etc.