Done both.
Singapore is probably better for the casual fan. Suzuka is for the purists.
Singapore
+ Track close to city, many MRT stops
+ Lots of quality entertainment post race at the music stands
+ Night race so most of day to self
Suzuka
+ Amazing track with massive elevation changes - I walked most of the track one day
+ Japanese crowd far more invested in race
+ Theme park and Ferris wheel included
Suzuka you really need to base yourself in Nagoya and catch the regional train (about 45min) in and out each day, then a good 20+min walk to track.
Did Bay Grandstand in Singapore, which has a great view of the city and sunset and is a fairly slow section with big TV screens, but limited overtaking.
Also got an invite into the Singapore Cricket Club on Day 2 which is nearer to Empress Grandstand and a fast spot.
Other expensive option is Singapore is some hotel room suites and the like with full catering (eg -
Singapore Grand Prix Viewing Only & Hospitality Package)
Suzuka I was in B1 stand with a great view down the Ss
F1 is always a good excuse.