There are too many variables to give an accurate answer. One variable would include where you live and the weather in which it will be used. Another is the type of steel that is used, bare, galvaneal, or galvanised. The preparation that the steel receives before it is used, and what type it is, will have a strong effect on how well suited it is for the elements.

The actual constructions that are used, how the aluminum panels are affixed and whether or not they are just taped on or mechanically fastened to the steel structure. The type (series) of aluminum that is used, will have a great deal of result on its longevity.

The maintenance, care and usage it receives by its owner, will greatly determine its long term usefullness. Just as all steel and all aluminum trailers can't be directly compared to one another, neither can the bi-metallic hybrids.

Aluminum over steel is used for one reason, cost. It's cheaper to build a steel frame with  lower labour and materials costs than one of all aluminum.

This can be a benefit to the purchaser, for whom the combination results in a competent and long lasting trailer. Stick with the name brands that have excellent records. Be aware that your results may not equal others, if you live in a rust belt, near to salt water, or decide to haul Bison instead of horses.

Take Gards Advice.

I'm in Utah, I frequently ride Antelope Island in the middle of the Great Salt Lake during the winter. It is accessed by a 7 mile long causeway across the Great Salt Lake.  I guarantee you there is salt on that road. Salt is plentiful and cheap, So it is used on all our roads during the winter.  I also tow to the local fairgrounds, and other places to ride all winter. Unlike your trailer, mine does see some salt that might cause rust.

My current trailer is a 2006 Logan Coach XTR.  It is a steel frame with aluminum skin.  Other than some rust on the axles/running gear and some of the gooseneck parts. I don't really see much rust on my trailer.