Given your options- I'd likely pick the Viper myself out of the two choices. Not to muddy the waters though- when it comes to the Athlon offerings, have you considered looking at the Cronus offering instead of the Ares?
The only reason I ask is you'd likely be comparing Athlon models within a similar price point as the Viper, you'd be giving up the option of reticle eyepiece options moving forward going to the Cronus but I'd say it's pretty good glass for the price and given the use case you mentioned- it may be more closer to an 'apples to apples' compression when adding in the Cronus spotting scope and if you don't care about swapping eyepieces and neoprene covers, you may be pleasantly surprised for the same money.
I'm pretty sure that I received an email or two this week that Athlon is running some kind of promotion presently that may make the Cronus option even more attractive for what you're looking to do. Just an idea- let us know how you make out regardless.
-LD