Which one is produce a good choice and give the best quality sound for
my room, 400 or 700 watt? I want to buy just single speaker. is
there any technical point here?
Product recommendations are probably out of scope of the SE-site.
But let me give you my opinion on wattage vs room-size.
For a 90 square-meter room, you are probably good with anything of 50 Watts and above. It is a reasonable small room, and your audience is only 100 people.
However it now going to depend on a couple of factors:
- Is your performance the main reason the audience is in the room?
- Are they all sitting/standing still, full attention to the performance, and no to little background noise, or noise pollution ?
- Or is this a bar, where your performance is more a background music and your audience are socially talking to eachother, hence your performance must at least over come (some) the noise the audience is creating ?
These factors are determining on how "loud" your performance must sound.
The two choices you give, 400 Watts / 700 Watts, probably have plenty of "headroom" to cater for anything like that, and are probably overkill to some extend.
The best advise I can give you is to go to a pro-audio re-seller and actually setup the gear in a room of comparable size. Test the equipment, and have someone else perform (or use a recording) to hear the sound yourself prior to buying anything.
Example I have been in situations like this, about 100 people, where the performance was not the main attraction of the venue (hence had to overcome noises generated by the attendee's, but not too loud to disrupt personal/social conversations) and I have used a single 400 Watt system, where the final amplifier would only work at 10-15% of its capacity (to describe it in a simple way, [edit based on comments] this is only to be taken as an example, not scientific method of determining power output of an amplifier)
"Your mileage may vary"