For Pro-Ac Tablette 10, 5 - 50W, 10 Ω... 86dB

With this forum it will be the Nova between those 2. However I'm sure someone will suggest you need at least a 300DR to power them suffiently. I'll say the Atom will work great with it assuming of course your room is on the smallish side. 

The 10 has been said to sound super whatever is behind it.

At least a 500DR, if not a statement! 

Otherwise exactly a Spurrier Sucks says.  But by far the best thing is to arrange a demo, listen to each, and decide if the cost of the Nova is justified for you.  It will sound better, but only you can decide how much that extra performance is worth.

thank-you... I'm pretty sure that the Nova will sound better, but my worry is that 80 Watt will be too much power for a couple of speakers designed for a 50 watt program... like a Ferrari engine placed in a Fiat 500... I do not want to create a system too unbalanced ...

The 80W Nova would be perfectly fine. There are people on here running them with a 200 or 250 with great success. A big amp with small speakers will work well; it’s doing the opposite where problems arise. You blow speakers up by overdriving them with an underpowered amplifier. 

More speakers are damaged by low power amps than by higher powered amps.

If you turn the volume up too far with a high powered amp, the speakers start to sound strained long before damage occurs, giving you plenty of warning and plenty of time to turn the volume down.

With too low a powered amp, the amp suddenly goes into clipping without prior warning; in this case the clipping immediately massively overloads the tweeter causing a very rapid failure.

Depends on the room too. My Ls50’s (about the same specs) sound great near field in my office with a Unitiqute v1. Not so much when they were in the living room with the UQ. Never sounded strained or clipped per se, just not enough grunt to sound cohesive. I think you’d be fine with either, but if your budget allows, by all means get the Nova! 

The Tab 10s are easy to drive, before my NAP250 I used a NAP100 with no issues, okay the 250 is better but the 100 was no slouch either the Atom or Nova will be fine.