One thing ReSpeedr does/did better than Twixtor 5.5 is with my GoPro mounted to the bike and the bike rolling down the trail. The UW angle GoPro lens has a fair bit of barrel distortion. This cause twixtor a bit of problems with some distortions around the periphery. ReSpeedr handles this significantly better than twixtor did when I tried it.
Respeedr can lose lock on its tracking of things, and their example videos linked on their site are applicable. It all depends on if something moves in front of the tracked object or a busy background behind a tracked object. I would suspect twixtor is probably better in this regard since it is an older more mature product. Also 3.5x more expensive so I would expect more on a value for dollar basis.
I personally prefer doing the resample outside of my timeline edit. This resampling is a time consuming process and I don't want that on my timeline. The Respeedr UI is pretty nice, IMO.
One thing Respeedr cannot currently do is a speed ramp. from X to Y over a given time. I have not used that but suggested it to them as a future feature. A simple workaround is to set your desired slow speed and do a longer stretch than needed and use a velocity envelope in Vegas to speed up, and dropping to 100% which would be your slowest speed.
Since the current twixtor now supports GPU it should be as fast as Respeedr.
Twixtor is waaaaay too expensive, UNLESS you use a lot of slo-mo. ReSpeeder is too much faffin' around as a standalone. When (if) ProDad finally get around to making this a plugin, I'll be first in the queue.
ReSpeedr has some advantages. For instance, it stabilizes as it slows down, which can make the slow motion very smooth indeed. Taking those occasional shots that you want to slow, and running them through ReSpeedr and adding it's renders to your media pool is a very efficient and easy way to work. I have them both, but am using ReSpeedr more.