It all comes down to money to be honest with you.
If you are looking to do monogramming only, you will be fine with a 4x4 hoop - those are by far the cheapest. You can get one for under $400 new. If you want to have a 5x7 hoop, you can get one for about $800-1200 (a Brother or Janome, etc) When you go to the 6x10 hoops, the price skyrockets and it usually includes the sewing features as well. Why they don't sell one that is reasonably priced that doesn't include the sewing feature but has the bigger hoops is beyond me.
You have to decide how much money you want to spend first - then start looking. Remember that you will have to spend about $100-200 or so for set up costs - stablizers and threads and whatnot.
I have a Brother 4500d and I love it.
I had a Janome 350e and loved it but wanted the bigger hoop - so I sold it and bought a Janome MB-4 which I detested... so I got the Brother and I'm happy as a clam now.