You may be making an out-of-order decision, so make sure you follow this process:
1. What do I want to do with this rifle?
2. What bullets do I need to be able to shoot to accomplish that goal?
3. Will I use optics, and if so, which one(s)?
4. What type of handguards/rail system do I need?
5. What configuration (barrel profile, twist, steel type, etc.) do I want/need to use my ammo correctly?
6. Which manufacturers can meet my needs?
If you don't know the answers to the first 5 questions, and know WHY they are what they are, then you need to do more research if you want to ensure you're getting exactly what you need.
There are FAQs and other important posts tacked to the top of most of the forums in the AR15 area that offer a lot of information to help you make these decisions, and for anything they miss, if you read back a couple pages in that forum, you'll probably find the answer you're looking for. If not, ask, and we'll help, but please read the FAQ posts before you do.
-Troy