Depends what you want to do. For local food and atmosphere the higher sois 60+ in Sukhumvit are good.

Siam Square is shopping mall centre but more pricey.

Riverside hotels have the best views with access to boats.

Honestly I would definitely say go to bangkok rather than chiang mai if you have to choose which one to go to. I would personally go to bangkok for two days and then take a day trip to Ayutthaya. My partner and i stayed in bangkok for three days and honestly we felt we should've stayed there a little longer. There's a lot to see and do in bangkok.

As for where to stay in Bangkok, it depends on your personal preferences. I have stayed on the river several times, basically due to trying out different river hotels to recommend to friends/acquaintances who might ask as a lot of folks ask me about hotels all around Bangkok. However, I am personally not a stay-on-the-river person and have never been. I am either a Rajadamri BTS-area person or having a stay somewhere between Siam BTS area and Sukhumvit 24 area.

I have stayed quite a few times in different areas around Bangkok over the decades. As far as I'm concerned, being really near a sky train station is #1 for me and also not having to waste time waiting for a boat to get along the river and then transfer to either a taxi or skytrain to get to where I need to go. However, every time I go to my dressmaker, located near Tha Oriental (river taxi stop) by the Oriental Hotel, I usually have to take the skytrain and then a river taxi of which I personally dread every time. Sometimes I just get off at Saphan Taksin sky train station and walk the 20 or so minutes so that I don't have to deal with a river taxi.

I suggest making a list of what you want to do if you choose Bangkok and then decide as to where you will end up staying.

Happy Travels!

I’ve stayed at places along the river and also not on the river and much prefer the riverside. And I love the express boats & ferries. Which isn’t to disagree with Guenmai, only to again point out how individual all these choices can be.

True the choices are all very individual. And for me personally if I had stayed on the river, back during my first trip to Bangkok, I would not have been a happy camper although I visited the river area to check it out. But, I was very glad to have stayed at the Siam Intercontinental Hotel on the land where Paragon Mall now is. Also my close friends who go to Bangkok regularly are not stay-on-the-river folks. To each his own.

Happy Travels!

My favorite was an upscale 3 room guesthouse, approximately across the river from the royal palace, in a room maybe 2 feet above the water, on an alley leading into a market with no vehicular traffic except for the occasional motorbike. When I pulled the plug in the bathtub I could hear the water hitting the river below me. The neighborhood and the room with long tail boats going by the sliding doors to outside made a huge city feel intimate and it’s when I came to love Bangkok.

If you've been to neither I would suggest Bangkok. It's definitely a major city and Chiang Mai is closer to nature even though it's still a smallish city. I wrote a trip report (we went a few years ago) but it might give you an idea of what to expect:
http://fanofblank.blogspot.com/2015/...hiang-mai.html

Personally I think there's much more to do and see in Bangkok compared to Chang Mai. And then from Bangkok you could also go to Ayutthaya as a day drip.

If you can only do one or the other, I would also choose Bangkok, like most of your responses here!
There's a LOT to see, do, eat, buy, experience!
If you really want to add somewhere else, you could look at either a day trip or single overnight to Ayutthaya, which is not particularly far, and will give you a different side of Thailand, a smaller city with a different vibe and some stunning temple ruins to explore.