I'd go for the largest trailer you can safely tow, hopefully at least 20'. Ideally 24'. Tim is correct in saying that the extra space is well worth the $$$$ if you can swing it. If you are using the trailer for racing you will need room for a tool box, generator, fuel jugs, jack, lights. It's not an absolute must, but there is the real possibility that you could outgrow an 18' or 20' trailer real quick, then you have to start all over. To get the best deal and save big $$$ you will have to travel to Indiana or down south to pick up the trailer. Check RacingJunk.com and look under the trailer section. You can get some sweet deals if you are willing to make the haul.
Just before the holidays I did some preliminary $$$ crunching for an open trailer, but checked out an enclosed trailer prices just for kicks. Franklin Trailers down on Rt. 73 was in the $6200 dollar range (basic trailer, no upgrades) for an 18' or 20' and Deande in Ewing is about the same. I saw 24' trailers (comparable features) for $4500-5000 on racing junk, but you have to go get them.
What are you towing with? It takes more truck to pull an enclosed trailer of comparable size.
From Racing Junk:
http://www.racingjunk.com/post/10175...ty-Hauler.html
http://www.racingjunk.com/post/93764...y-Trailer.html
http://www.racingjunk.com/post/95611...l?free_trial=0
What kind of open trailer do you have?
Chris