As with most "what tool should we buy" threads that show up here on CD, I have an opinion so I guess I'll chime in on this one. First off, the type and size of miter saw you buy will depend on what you plan to cut. For FIRST use I assume you will want to use it to build practice field elements and maybe cut some light aluminum stock. Over the years, the majority of the stuff you would cut on a miter saw for constructing a practice field could be cut on a 10" non-sliding type saw. If you have the $ and space and don't mind the bulk of a 12" saw, then by all means go ahead.
I personally own 2 miter saws. A "good" one and a "cheapie". My "good" saw is a 12" non-sliding single-bevel Porter-Cable model almost identical to
this one but without the laser. I bought it several years ago before laser lines were all the rage. It is a really solid workhorse of a saw. I have cut about a zillion 2x4s, 2x6s, 2x8s, 4x4 posts, 3x5 landscape timbers and even a few 4x6s over the years and it is still going strong. I have just completed a large deck using it to cut every single post, rail, and deck board because every board had to be individually cut to fit because the deck was not square or rectangular but sort of oddly shaped to fit and existing space. My point is Porter-Cable makes good stuff built to last and do the job. Our team has also used it extensively during build season for the practice field and to cut about another zillion feet of aluminum stock. We used a lot of the 80/20 Aluminum QuickFrame material on our 2006 robot and my Porter-Cable cut (and re-cut) every inch of it. 80/20 QuickFrame is basically like 1" box tubing with about 1/16" wall. Cutting aluminum stuff like the kitbot frame size material is also no problem. Cutting round stuff is another story. A while back I discovered that it isn't really a good idea to cut round tubing on a miter saw. I was trying to cut some thin walled aluminum tubing to use as spacers for the 5/8" keyed shaft in those black plastic helical gear boxes FIRST supplied in 2003/2004. Besides the fact that I couldn't get good cuts, it was just plain dangerous. A piece of cut tubing somehow got shot out of the saw like a bullet, went zinging past my head, ricocheted off a trash can behind me and flew about 150 feet across the street (I had the saw setup in my driveway) and landed in the neighbors yard after bouncing off their house. My point is be very careful using a high speed saw to cut aluminum. With a carbide tipped blade the saw will cut it fine but once the piece is cut in two the problems can happen very quickly. I have no problem cutting flat stock up and rectangular stock within reason but I will never cut round stock on it again. The largest aluminum stock I have personally cut was probably 1/2" x 2" 6061 flat bar. Also, don't try to cut anything too small like say shave a 1/4" off the end of a piece of something unless you are fond of the idea of bullets flying past your head. (It goes without saying but I somehow feel compelled to say it anyway, never, ever, ever, cut anything, especially metal on a miter saw without your safety glasses on and make sure there are no bystanders nearby or in line with the saw cut.)
Now, my "cheepie" saw is a 10" sliding compound from Harbor Freight. It was $99 on sale and is similar to
this one. It is smaller and lighter than my 12" Porter-Cable and is easy for me/one person to move around so I usually grab it if I just need to cut a few small things like 2x4s or 2x6s. The quality of construction is definitely not anywhere near that of the Porter-Cable but it can cut a 2x12 and my 12" non-sliding Porter-Cable can't without flipping the board over. One interesting thing though is the cheap Chinese saw from Harbor Freight was spot on square right out of the box and has remained that way for the several years I have owned it. When I bought the Porter-Cable and opened up the box it was way out of alignment and I spent hours getting it right. During my recent deck building project I noticed that it has again gotten out of alignment and I am dreading spending the time to re-align it so that all the angles are true. The other major differences are that the "cheapie" 10" HF saw doesn't have near the power that the 12" Porter-Cable does (duh) and that the HF saw doesn't have an electric brake on the blade which makes it somewhat less safe to use than the Porter-Cable. All in all, I am very happy with both but each has it's use in my shop.
Now with all that said, if I had it all to do over again I would buy a DeWalt dual-bevel saw like
this one or if I had the $$$ the
sliding one like this one. The dual bevel DeWalt saws place the motor behind the blade so you can miter and bevel in both directions. In practice you can usually get around the limitations of a single bevel saw by flipping the material around but sometimes this is very awkward.
So, bottom line, if you are limited on funds and don't need to build a whole house I personally think the $99 saw from Harbor Freight can be very useful to a FIRST team. If you have the $ and prefer a solid built tool that will last forever then stick with the DeWalt. Don't get me wrong, I love my Porter-Cable and either it or it's brother made by Delta (same saw - Delta and Porter-Cable are the same company) are great saws but in hind sight I now see why all the contractors use those yellow Dewalts.
For the record, I'm not connected to any of these tool companies and own all sorts of power tools from Craftsman, Porter-Cable, DeWalt, Bosch, Harbor Freight, Black and Decker, Milwaukee, Ryobi, and others that I can't think of right now.