I have had both and will give you my reviews of each
1) Horsebox, there isn't a 3.5ton box that will take 3 horses, only recently has there been a 7.5ton box designed and on the market to take 3 horses so putting three in a 3.5t could invalidate your insurance, get you points, a fine and not being allowed to carry on your journey.
Though a box may have room to take 3 horses it is not legal.
You really need to find out before buying.
Okay
Positives:
Ideal for transporting horse with extra's such as carriage, tack, hay, feed , water etc.
You can travel together with friends
Some with living mean no hotel costs if staying overnight at a venue.
More room for horses.
More secure when transporting
Downside:
Tax, Plate and insurance required,
Servicing can be costly (ours one year was £2000)
Replacement tyres, disc brakes are expensive
Can't go anywhere once at a show (unless someone brings a car)
More expensive to park on livery yards
Not as easy as some make out to drive at first due to size
More expensive to purchase
Steep ramp can deter many horses
Can be hard to sell, value drops quickly.
TRAILERS
I have had two, sold first one as I bought my horsebox due to starting Horse Driving Trails. The second I still have now. (bought recently as no longer driving and box was too big and expensive to keep for the few times we now used it).
I have always bought Ifor Williams, I love the aluminium floors, stability, design and comfort
POSITIVES:
Easy to tow
Cheaper to park at a yard or small enough to park at home
Low ramp for nervous horses.
Partitions can be taken out (I never use my partitions, horses preference)
No Tax or Plate required
Service and insurance much cheaper
Great for teaching trailer loading
New designs make for better lightening inside (more welcoming for horses)
Can be unhitched leaving you free to drive to places.
Easy to maintain (less can go wrong than with a box)
Keep there value (Well Ifor Williams do for definite)
NEGATIVES
Easier to be stolen so plenty of lock deterents required
Not as stable as a horsebox
Can fishtail when towing so driver must be confident / experienced to tow
Some horses don't like the low roof (compared to a box) but good training and patience over comes this.
Off the top of my head that's it, will prob remember other things as the day goes on.
What comes to my mind is buying an old horsebox on a budget often leads to many parts being needed each year at a cost.
Its advisable to get breakdown cover so that adds to the cost.
Box or Trailer, only you can really decide, it also depends on how much its gonna be used, if staying overnight, if other items need transporting that are big (such a quad bike, carriage etc).
Don't go for a horsebox because you may be asked to take friends horses to places.
You can't charge for this so it will be at your expense and its a lot of money to pay each year just to have a horsebox.
Also its not jsut a serive once a year, what if something else goes wrong during the year, money soon adds up.
what would be more practical?
I miss my box, I bought it, had it modified with side stabling and had it fully resprayed and my personnal logo added. Total Cost = Approx £17k then recently sold for £8k and hate to think what we spent in fuel, services etc.
However I love my trailer and its so easy to hitch up and go.
Hope that helps some
Demson