I do distance riding with my Morab.
I finally got a HRM this fall, after borrowing one from a friend this spring. It might not be such a bad idea to borrow one, if available, and see how you like it.
My friend got a V-MAx, I got a Polar. I have no issues with my Polar- she has trouble regularly with her V-Max. Of course, EVERYTHING varies when it comes to horses!
Slypner Gear sells the Polars- the whole kits. They make it very easy to buy.
I talked to a bunch of distance riding buddies before I decided to get a HRM. Some preferred NOT to have one, for they wanted to rely on what their horse told them, and not a HRM. What it did for me was show me that I was not working my horse HARD ENOUGH- his rates were great, and showed me I could push him harder than I was. Our conditioning was much more successful after that. I did not use it all the time- a lot in the spring for legging back up after winter. And at times to check him out, and usually durng the actual rides.
AS for buying one- I kept an eye out on Endurance.net, and finally found a used one and saved $50. It was worth the wait!
Edited to add: I bought the Garmin 201 as well-used, great shape. I do wear it on my wrist. My HRM watch is attached to the swell of the pommel of my Tucker! Like a little dashboard!