Depends on what you want, really. For starters, Race-Tech emulators are the original product, designed, produced, and patented in the USA. They're a known factor. Mike's are knock-offs reengineered in either Peoples Republic of China or Republic of China (Taiwan), I'm not sure which. The only part that might differ from the Race-Tech product is the preload spring, but it's critical to the functioning of the valve. To date I haven't been able to get test comparisons of spring rate and durability. Race-Tech also supplies detailed installation instructions, setup suggestions, and tech support. Mike's does not.
Re. springs, emulator valves are designed to do well what dual-rate springs do badly, which is to modulate response of the forks to different sizes and profiles of bumps. They perform best with straight-rate springs selected for your weight, your bike's weight, and your riding style. Off-the-shelf straight rate springs aren't available in the diameter we require, but Traxxion Dynamics will suggest a rate for you and produce a custom-wound spring for less than you'd expect--the price was around $125 a few years ago, and I doubt it's significantly more now. If the springs don't deliver the right sag in a viable range of preload, they'll have another pair wound and sent to you with shipping paid both ways--can't beat their customer service and ethics.
Well I know this thread is a decade old but I'm going to shoot my shot in the dark anyways! I'm in the midst of building my first xs650 and have seen this ^ recommendation from a few people as I've been digging through the forums researching things. So I reached out to Traxxion Dynamics and was basically told that they don't have any build notes for the xs650 and they've never built springs for them before ... Can anyone shed any light on this for me? I think the race tech emulators and straight rate springs are the way I want to go. Just need to source components.