the 50 and the 70-300 would be my preferred lens to take i would use mainly the 50 if they are in an arena or the 70-300 if they are in an open field for the riding shoots and the 50 for the portrait shots
i would try and keep your shutter speed above 600 for moving shoots to freeze the action. or 300 if you want to go for the slightly motion blurred pictures for effect.
portraits
you want both horse and rider to look relaxed and happy, take a crisp packet or keys with you to make a little bit of noise or throw(carefully) to get the horses attention its much more appealing is the horse has his ears forward. if the rider and horse are facing you make sure you get both the horses head and the rider in focus as it can detract from the image if you have the rider in focus but the horse out of focus, most importantly have fun with it this will relax you and rider, make sure to get a clean background with no clutter.
Ridden shots
be careful on the horses leg position as that for me can make or brake a photo for instant
at a walk try to have all 4 legs visible a low head and ears forward not always possible depending on the horses nature it may not want to put it ears forward and while the horse is in motion you can't tempt them forward either but try you best to capture the horse when its ears do go forward.
equine portrait by
Aralia photos, on Flickr
at trot you would want the horse to be fully extended looking relaxed and the rider relaxed concentrating and both happy for example
lottie showjumping practise by
Aralia photos, on Flickr
tara and arnie by
Aralia photos, on Flickr
this shows off the horses movement and looks more pleasing where as a show like this
tara and arnie by
Aralia photos, on Flickr
doesn't show of the horses movements makes the photo look a bit awkward and less pleasing to a rider.
the canter again leg position of the horse is the key you want to try and capture the horse just as the he arches his front leg and sits back on his hind
kelly by
Aralia photos, on Flickr
jumping
you want to aim to capture the horse right at the arch of the jump just as his toes leave the ground, also one i find increasingly popular with show photos if the jumps are bigger are slightly later into the jump just before the horse starts to to come down from the jump
just as they have left the ground
mel and sam by
Aralia photos, on Flickr
lottie showjumping practise by
Aralia photos, on Flickr
then the slightly later shot
showjumping by
Aralia photos, on Flickr
getting the timing right with horses can be difficult but once you get the timing you can become pretty consistent and shooting on burst mode can help if you are struggling
hope this helped slightly I'm by far a pro but these are the type of shots that i have sold at shows