We tried an endless, swimex and the riverflow. We ended up with a riverflow for a few reasons.
1) You can integrate it in the main pool and use it for other purposes, like kayaking.
2) It's silent
3) It's maintenance free
4) The flow design drives a more natural swim posture than the endless. The endless makes you swim at a 4-5 degree angle, with your legs and behind sinking a bit - this is due to the location of the intakes vs. the exhaust.
The swimEX is a little better swimming experience because it really feels like you are in a river of water. The cross section of flow is massive and more linear compared to the riverflow and endless. (though very noisy) Our big issue is that it's a big hottub and super expensive. The riverflow is around 15K for their top of the line unit on top of the swimming pool and the swimEX is close to 60K for the system integrated as a hot tub.
Two physical therapy centers in Austin with both riverflow and swimEX systems recommended the riverflow for maintenance reasons.
One word of advice on the riverflow - do not work with the Florida office. Our experience has been unbelievably bad. Our pool has been delayed several months due to parts delays and incorrect parts being sent. The rep in Florida is very hard to work with through the pool contractor. I've had to call the CEO our of California several times to get service or questions answered. Send me a PM if you want data points on my experience with the Florida team.