I’m not exactly sure I would say that eventing is definitely more horse friendly, at least in terms of work load. Eventers work very hard between events. 6 days a week (some even 7), and at the two star and up levels, they often have multiple days of fitness (maybe a trot or swim day and a gallop day. Often they gallop every 4 or 5 days as their fitness demands get greater). They school on the flat, which is very hard work at that level, and most jump at least once a week (via social media, it seems to me some UL riders are certain their horses forget how to jump or go xc between events and need massive schools to stay sharp). Save three days, most eventers will go right back to the grind after an event with maybe a couple of easy days (and I’m seeing less and less time off after three days, nowadays). It is HARD work being an event horse. I know less about jumpers, but as I learn more it seems they might do as many days, and even some fitness work, but a lot less really hard work that I see eventers do. Lots of flat work to keep them limber and broke, but a lot less jumping on a weekly basis. Seems that once a jumper reaches the 1.30m level, they rarely do more than a gymnastic at home and only jump a couple of courses a show. I will fully admit that I have not been entrenched with jumpers like I have with eventers, but it is my casual observation through various sources.
An eventer may stay at home more and not travel as much (no multi week shows), and they may or may not get more turn out (depends a lot on the barn. I’ve been in UL barns where the horses rarely see the inside of a stall. But I’ve also been in UL barns where they only go out a little at a time). Lord knows both sports will use all sorts of veterinarian intervention to keep a horse on the road. But an eventer’s life is no walk in the park and can be both physically and mentally taxing. Since you have three phases you have to be an expert at, you have to do a lot of work to stay sharp.
As for how many starts, it depends on the horse and his rider. I don’t know many who’ll go more often then every two weeks (at the prelim level my horse was proving that he did best on no more than every three weeks). Also keep in mind there is not an abundance of UL events compared to jumper shows. It takes some considerable strategic planning to get enough runs for an UL horse and rider, particularly if you aren’t on the east coast (or, more specifically, in the mid Atlantic). Some people have the luxury of experience that they can be even more conservative and only running their horses for choice events (you see this most often with 4 star horses, particularly in England, where a good 4 star partnership may only show up at the premier horse trials between major 4 stars).
Interesting questions! Eager to see what those who HAVE been very much in both worlds have to say.