Pretty spendy to buy one, but it's hard to beat a Stur-D tub. http://www.stur-d.com
IIRC, a local place here rents out a tub for $150 day. Not many guys own one, but lots of guys rent one to do fall shots. It's nice to set up on fresh clean pasture with little to no dust or mud like a permanent corral has at times.
I'll second the stur-d, we bought one this fall and there extremely sturdy as the name implies, we have had crazy cows that you can't hold in panels but the stur-d tub held them no matter how hard or high they jumped, the tub is one solid unit so it isn't going to break like I have seen others do. If you decide to buy one I would really recomend the stur-d. I also like the adjustible alley, if you are working calves one day and cows the next all you have to do is lift 4 bars and push in. The head gate isnt to bad either, it was a little stiff when we got it but after runinng about 150 head through it, it loosened up so it works pretty good, the only negative comment I have is that when your branding shoulder branding yearlings it can be a little hard to get at the shoulder.
We have a Scott portable alley and tub. They used to be made up in Soapweed's country but have moved I believe. One of the most problem free pieces of iron I ever owned. Heavy built yet moves easy down the road. Adjustable alley, and a couple of neat little gates that give you access to the neck. My vet likes it enough that he started telling customers about it and I have started renting it out.
I've got a Rawhide portable corral...they are very nice where you truly need portable...If you don't absolutely need portable I would still invest in a permanent setup.
The Rawhide is easy to set up, trailers at highway speeds, and is very sturdy. All those things make it great for when the need is absolute portability. If you have multiple spread out pastures, pastures spread out miles apart, etc...
The downside to the rawhide is that sorting is difficult at times unless your cattle are very docile. Also, when the rawhide is setup in the alley format there is no way to work a cow through a headgate/squeeze and back into the corral. We ended up building a weaning pen about 2 acres in size to contain the cattle we wanted to catch after they had been worked...which somewhat defeated the purpose of the portable corral. We have used the rawhide for about 3 years now...it is very sturdy!