TBH it depends on the lorry itself. An E reg is obviously younger, but if its been hammered and has millions of miles on the clock and it generally run down and you are comparing it to a B reg thats never exceeded 50mph and been housed in a barn all its life.....not helping am I? LOL
You really need a mechanic to see both lorries and give you a full report...some are workhorses and keep going for years and years, others are a real money pit! There is no way of knowing from these details, it may be that both are bargains or that you shouldn't touch either with a bargepole!
In all honesty the focus should be on rot ( Rust ) at that age of the chassis its self - it could be mechically sound ie engine , gearbox but all of that is no good if the chassis is crumbling away.
Metel corrodes over time , when you are looking 26 year old lorry the chances are there will be quite a lot.
No one can help you with that decision except an expert in checking horse lorries. Try tinging teh organisation of horse box and trailer owners, they will be able to give you the details of someone near to you who can carry out a thorough examination of the box for you, both from a mechanical, chassis safety/soundness and container safety. yes it costs to have that check done but far safer than your horse potentially falling out the bottom or the chassis splitting in 2
Would strongly recommend you get the prospective purchasers checked out by a reputable mechanic. A while ago, we were considering a very nice looking Ford Cargo, 3 horse with living and sent in the Organisation of Horsebox and Trailer Owners Mechanic. Good job we did as it had a list of things as long as your arm long with it and the inspection fee is the best money we ever spent. The saying don't judge a book by its cover is very true. Good luck