I am upgrading my bike which is exciting but I can't spend too much. Do I sacrifice carbon for aluminum if that gives me the option to go full suspension. Do I go for a solid carbon hardtail that may sacrifice some of the trail comfort for the durability. How much of an advantage is the full suspension over hardtail if you can only buy one for $1500?
At 1.5k it has to be a hardtail. Rear suspensions need to be done right to be worth having in the first place. If I was a long-travel guy (I'm not) I would start looking at FS bikes once my budget passed 2k.
As to the frame, 1.5k would get you a cheap/heavy carbon frame or a top quality aluminum frame. I'd go aluminum.
The above assumes that you are buying new... I would consider buying used.
If I were you, I would get a used aluminum FS. I wouldn't mess with a used carbon bike, that can be risky.
Brand and model really doesn't matter if you know your components. If you have no clue how to identify good components, you should stick with big, well-known brands like Trek, Cannondale, Giant, Specialized, etc.