Correction - ALL Wormers DO NOT contain the same ingredients!!! NOR do all wormers kill the same worms.
The powder wormers which seem to be still popular in the UK often have a single active ingredient which is effective against a single worm type only. This means that you may have to worm quite frequently with different powders to target - roundworms with one powder, tapeworms with the next etc etc and you will have to keep track of the different cycles.
Modern modern multi-wormers are usually paste, it is particularly important to be aware of the chemical you are using (not the Brand name) and rotate the chemicals on an annual or bi-annual basis to ensure that the worms do not become resistant to a single chemical type. Multi-wormers include ivermectin and moxidectin but these may still miss one type of worm, so check.
There isn't anything inherently more effective about paste vs powder - provided that it is the same CHEMICAL ie ivermectin paste vs ivermectin powder.
What's important is that the horse gets the correct dose. You can imagine that it may be easier to give your horse a single correct dose of paste once every 8 weeks rather than make sure he eats all his powder for 5 feeds! Esp if he throws his food around - how much got in? But that's a management issue and will depend upon you and your horse to decide.
A good trick for horses that don't like the paste - get a plastic syringe from the local chemist and fill it with molasses,sugar water,apple sauce or something he likes and give him a squirt each time you feed him for 10 days before his worming's due. By the time you swap it for the wormer - he'll open up and swallow like a pro!
For other horses, like mine, who can't imagine that you'd offer them anything other than food - a little treat afterwards will make up for any unpleasant taste.
For more information on worms and wormers check out
Virtually Horses Worm Feature Article