I'd base my buy on the kind of riding I would be doing ie, cross country, all mountain, freeriding, downhill, and urban.
Then I would decide what kind of rear suspension and how much travel to go for. There is single pivot(which I have), Horstlink and variation of 4 bar linkages, vitual pivot designs etc.
3 or 4 inch travel bikes are for cross country. 5 to 6 inch travel are for All Mountain or light freeride. 7 inches on up gets you into freeriding and downhilling. All of these diffrent setups are for specific riding conditions. Get what you need.
Of course your budget will determine what you can get and what you can't. It is easy to drop a lot of money into this.
If I was just starting out and know what I know now I would get a 5 or 6 inch bike. I also am a big guy and need a sturdy steed. I ride xc, all mountain, and do some light freeriding/downhilling. 6 inches will get you there if you ride like me. If I did just xc at my size I would get a 4 or 5 inch bike.
We have alot of climbing here in Washington so I factor that aspect in. I like a full suspension bike to weigh around 32lbs preferably on up to 36lbs for the riding I do. I weigh 220 and am 6' tall so I need some strength regarding the bikes build. Lots of things to think about. More than I want to write about lol. Take your time and buy wisely. There are so many choices out there.