15Mbps is good enough for most things, apart from high quality 4k streaming. But note that location is important. Some places you'll get the full 15Mbps, whereas in others you might not.
If you live in the Pattaya area, people in Jomtien/Phra Tamnak might be better off with DTAC, whereas in Central Pattaya fare better with AIS. The only way to know at your particular location, it to test or ask near neighbours to do so.
Since you already have a Truemove H SIM, you can buy a one day pack and run speed tests via https://testmy.net where you might use it. https://topping.truemoveh.com/packages You can buy a 100MB test package for as little as 10 baht there. or spend 29 + VAT for 2GB for more extensive testing.
The advertised speeds are 'up to' rather than minimums. But if you are in the right location, you can get close to the advertised speeds.
Another thing worth a mention, is that AIS is the only major network to have something close to a symmetrical offering. In other words if they sell you a 20Mbps package, that it may have up to 20Mbps downloads and 20Mbps uploads. DTAC might give you 20Mbps down, but only 4Mbps up. Truemove H, I don't use.