I agree, the Crowne Plaza does seem like the best option for this length stay (either that or a hotel in the city) even though it does require going through immigration. The airside hotels are more basic and really more suited to 6-8hr layovers.
Immigration shouldn’t take any more than 30mins arriving into Singapore, and departing no more than 5mins (departure mostly done at automatic gates). Security is at the gate your plane will be departing from, so you need to go through that regardless of where you stay, although queues are usually quick (best to stay in lounge as long as you are comfortable with - no need to be at gates early like in the US, I usually would recommend reaching gate 30mins before departure for those who like to be their early and 20 mins for those who are OK with getting there late).