Originally posted by Captain REX
The ARCs have a much higher level of training though; the Commandos received their training, in a milder version, from them.
According to various sources, including those cited in Wookieepedia, the Advanced Recon Commando (ARC) is superior to the Clone Commando (Republic Commando).
This is not to belittle the RC, as they too are very deadly, but the ARC enjoys a number of advantages, including personal training from Jango Fett, and complete operational independence free from any genetic modifications to induce compliance. The RCs, however, merely have reduced genetic modification.
The details about the history and differences between the ARC and RC are described under the History section of the Clone Commando article.
Sources:
http://www.starwars.wikia.com/wiki/Advanced_Recon_Commando
http://www.starwars.wikia.com/wiki/Clone_commando
Based on what I've seen in the shows, played in the games, and read in the books; I'd have to hand it to the Commandos.
ARC troopers are portrayed to be regular Clones that have distinguished themselves in combat and received additional training, armor, and weapons in order to be sent into and succeed in SpecOps missions. Commandos were bred and trained from the get-go for Spec/BlackOps missions. They are also squad-based, although they can act independently, their true strength is in their team. They also have better armor, weapons, and most likely more experience and training. It is more expensive to create and outfit a Commando than an ARC trooper which is why there are more ARCs than Commandos. Chances are, as Commando casualties increased, more ARCs were trained. (For Halo enthusiasts, think of the difference between Spartan IIs and Spartan IIIs or ODSTs)
The ARCs most likely rolled over into the Empire as Dark Troopers, and the few Commando units left were kept in service until they died or their units dissolved and they probably became officers or Dark Troopers themselves, but I digress...
I think you'd need a lot more than two ARCs to take out a Commando squad. One-on-one would be a bit hard to judge, but I think a fully outfitted squad of four Commandos would be even against 10 ARCs at most.