GT and TREK mountain bikes.
 

Which bike is better GT TALERA hardtail or TREK 830 ANTELOPE hardtail. The GT is 20" the TREK is 18". MY height is 5`7" which size mountain bike is good for my height.

The Trek would fit you perfectly.

I agree, the Trek will likely be the best fit. Both are decent bikes, btw.

Op, what you didn't mention is the age/model years of the bikes in question.

On an older mountain bike, both 20" and 18" frames would probably be too big for you to ride off road, because the clearance over the top tube would be too short.

On a mountain bike being ridden off-road, you need a minimum of 4-5" of clearance over the top tube when standing on the ground with riding shoes. That's to make sure that you don't injure yourself if you have to get off the bike in an emergency.

I'm 5'10" with shorter than average legs, and I ride a 16.5" 94 Trek 930 SHK. 18" Trek mountain bike leave me with too little clearance over the top tube. 18" frames are too big for me. At 5'7" unless your pant inseam is longer than 32" or so, the 18" frame is going to be too big for you.

Of course, if you're planning on riding on-road all the time, many people size mountain bike frame more like road bikes, and a 18" or 20" frame should fit you fine. You need to go out and try riding both of those bikes to figure out which one fits you better.