There is something magical about the river location for FIRST TIME VISITORS to Bangkok. For this reason, also consider the Royal Orchid Sheraton.

The 2 pools are a plus.

But, I'm hoping you can clarify your #3. Sukhumvit is walkable to dining and bars, but I'm not sure they'd be the type of "Dining" that you're looking for.

If it's Fine Dining, there is a multitude of options in the 4 and 5 star hotels all over the city. Both Sukhumvit and the River have these type of hotels, but I'd suggest that the river location would win out for 'great fine dining'.

If it's just authentic Thai food (both dine in and street food),lively street scene (people watching, street markets, plastic chair bars and restaurants), bars, nightclubs, women, etc. then Sukhumvit will do you fine.

Bottom line, I think you should split your time between "The River" and "Sukhumvit", and the first half should be by the river. Make the Royal Orchid be your decompression point, it has great suite upgrade percentages, a good evening cocktail service in the lounge at night, the river location allows you to FEEL how Bangkok is different than other cities. Attractions are easy and unique to commute to/from by the river... 2 great swimming pools.

By the time you've settled on the riverside, you're ready for the sensory bombardment of Sukhumvit. MANY SPG hotel choices in that area. The right hotel for you will depend on many things, including your budget and whether you are paying cash or using points.

good luck.... and don't forget to report back on what you decided and how things went.