Snowy-bellied vs Rufous-tailed hummer ID

Looking at the pics in the Costa Rica and Panama books, a person wouldn't expect Snowy-bellied and Rufous-tailed Hummingbird to be difficult to tell apart.  That assumes of course that a person sees the gleaming white belly of the Snowy which is prominently displayed in the plates.  The problem of course is that hummingbirds are happy to turn their back to you and don't show much of their belly while nectaring at flowers.  The first time a guide called out Snowy-bellied in Costa Rica I was really confused since it looked like a Rufous-tailed to me.  Even after more experience with the bird, I still initially mis-called these Snowy-bellied's at our lunch stop on our first full day in Torti.

What would you call this bird?

Or either of these next 2?

 All have extensive brownish on their back and tails, but all are Snowy-bellied.  There's actually more dorsal brown on Snowy-bellied than this next Rufous-tailed.

The Rufous-tailed has much more intense rufous than the browner Snowy-bellied.  This next Rufous-tailed shows some extra terminal dark with the tail spread that the Snowy-bellied's don't have.

That would be hard to see in real life without the bird frozen by the camera's shutter though.

So how to separate them?  The bill is helpful.  If you look carefully, the Snowy-bellied's looks a little shorter and straighter.  It's also blacker.  Rufous-tailed at most angles shows a quite pink bill; Snowy-bellied has limited pink to the lower mandible.  First Rufous-tailed...

 ... then Snowy-bellied.

As mentioned earlier, Snowy-bellied has some pink to the underside of the bill, but any view of the underside of the bill reveals the brilliant white belly.

One last look at Snowy-bellied in reasonable light.

The first take-home is to be aware that these two birds at many angles look quite similar. The 2nd is to study Rufous-tailed closely; it's way too easy to not look carefully at this common species in the excitement of looking for new birds in the tropics.