It is mentioned on the Star Wars wiki page, that most of the Havoc Squad was killed. If you speculate about the size, even though it is only squad by name, it could consist of more than just 6 soldiers. I haven't read any of then TOR novels, but I doubt that Havoc Squad is the only SpecForce unit in the entire Republic.
General Garza mentions it is the 325th SpecForce unit IIRC and it is codenamed Havoc. So in theory there could be more than one SpecForce squad.
On topic ARC troopers and all the clone troopers are clones, genetically engineered to perform at top efficiency and to be cannon fodder. ARC and Delta Squad have received additional training. Havoc fights for the survival of the Republic, while clone troopers just follow orders (look up Order 66).
My choice, Havoc Squad over ARC troopers
Yes there is more than one Spec Forces Squad, You run into a few during the Bounty Hunter's Story line on Balmorra.
That said I'm going to go ARC Troopers. Your a bit wrong on them receiving additional training, unless your talking about the the ARC Troopers who were originally Clonetroopers who got promoted to the position like Echo and Fives. I'm sure the ARC Troopers the OP is talking about are the genetically bred variety.
ARC Troopers are genetically altered to be basically "Raw Jango" They can be independant and can work as a team if need be. And they can question or bend orders if they see a flaw. Essentially they are Jango Fett except they are loyal to the Republic. Even willing to sacrifice other clone embryos during the Battle of Kamino if they were in danger of falling into the wrong hands. Comparing them to a regular Clonetrooper is sort of an insult.
Lets use Fordo's Rescue Force as an example.
This squad has Two Heavy Gunners. a Rotary Cannon and Quad Laser which technically brings the fire power to 5x that of Jorgan and the Commander rolling Commando combined. They also have maybe 6-4 Lieutenants packing Westar Assault Rifles equiped with Grenade Launchers. Fordo is packing Dual Pistols with an electroshock cord I figure they could handle Jorgan and the Commander pretty well.
For 4X they'd simply unload all their weapons into him, Grenade Launchers and all like they did Grievous, and 4X isn't as Fast or Agile as Grievous meaning he would get caught in the blast. And if that didn't do it they'd call the Gunship in which would blow it's way in and Vaporize 4X. and really any member of Havoc Squad in sight.
That...and the ancient tactics that Havoc Squad used were most likely flash trained into the the ARC Troopers during their gestation period. Giving them a tactical advantage.