Glas or Alu......when that uber deal presents itself (shop hard) go for it.
I'm a tin guy but I do like glas as well. Lavro was my first DB ride.

As stated above, trailer quality is very important. You will see all manner of trailers in your quest. Avoid anything that looks like homemade, converted, bolt together or thin wall crap.
A bona fide, industry-standard DB trailer like a Baker will cost about $1500 new. There are other trailer brands around that are similar to a Baker......look for those qualities in your trailer.