There are different styles to consider, 2 handed handle style or 1 handed palm style, I much prefer the palm style. There are also different throws for more or less aggressive sanding, say 5/16 or 3/8 for removing stock, 1/4 and 3/16 for all around, and 3/32 for fine finishing. It depends on what you are trying to accomplish.
Personally I have been running the $50 HF model for years, dont really take care of it and it has been just fine. I have used both the hook and loop and stick on pads. You can purchase new pads, they are a wear item. I put a good 3M pad on the HF unit and it was night and day. I also dont recommend going cheap on the abrasive, the cheap ones that do stick will tear when trying to remove them and leave paper stuck to the disc. Good ones peel right off.
I dont really reuse abrasive, I found my time was worth more than a new disc and running a disc longer than it should wastes time. I dont have any complaints about a hook and loop, I just like the feel of the stick on style more. The interface with the material you are sanding feels more natural to me.
As I said I have been running the better HF model. They have a cheaper vacuum style one that was nowhere near as good that I started with. When I buy a new sander for the metal shop it will likely be a Dynabrade or 3M. You can pick up used ones on ebay for less than $100 and the airmotors can be rebuilt in them. I will probably get a 3/16-1/4" throw.
There are also different horsepower's to consider, but I dont have any advise there. I assume like other air tools, higher the HP, the more CFM and louder, but the more torque available for hogging.
Another thing to consider is non-vacuum, vacuum, and self generated vacuum. I have tried both vacuum and non-vacuum and although the vacuum style was cleaner it was more cumbersome with the hose, louder (vacuum running), and one more step to setup. The abrasive is also generally more expensive. If anyone has an opinion on the self generated style I would like to hear about them, maybe the middle of the road between the two???
Mike