Most of the time (in a 1v1 simulation anyway) by the time you manage to get behind the Onslaught with your Conquest you will be clinging to life. Though the Conquest is indeed more manuverable, it isn't so much of a difference that you can loop around outside of the Onslaught's range and then cut in behind it. No, you have to burn past it at point blank range and THEN use maneuvering thrusters and cut in behind it hard. That will work, but most of your weapons will be offline from the pounding you took in getting there. This assumes a 1v1 in a sim where the Onslaught isn't distracted by anything else of course. Real battles are totally different and the Conquest is great for the mobility it offers. As Gunnyfreak said, it's great for controlling the battlefield vs smaller ships. I voted Onslaught though; being able to tank hits on the armor with no need for shields is very satisfying. Not to mention the firepower.
Right. That, and all things being equal with both the Conquest and Onslaught outfitted with ITU, the built-in Thermal Particle Cannons have extraordinary range that'll prevent the Conquest from getting behind the Onslaught without first taking large amounts of damage and/or disabling many of its weapons. In a capital v. capital 1-on-1 fight, the Onslaught is tough to match, even with it in AI hands. That said, its extraordinarily slow speed, low maneuverability, and high deployment cost make it pretty tough to deploy economically.
Nevertheless, I voted for the Onslaught.
Maulers have the same range as the TPCs and the Conquest has the advantage in maneuvering. If the Onslaught ever uses his burn drive while the Conquest is sitting at the maximum range of his Maulers as he should, the Conquest will dodge and the Onslaught's TPCs become irrelevant. The Onslaught dies to a Conquest behind him.
If both players play perfectly though, it'll be a fight between their respective shields' ability to withstand the enemy's Maulers and TPCs (the latter mostly missing IMO), with the Conquest deciding whether he wants to move into large ballistic weapons range (optimally Storm Needlers) in which case the Onslaught has 1 or 2, the Conquest has 2.
The Storm Needlers force the Onslaught's shields off otherwise it's a near instant overload. The Maulers disable his front facing weapons.
The Conquest has higher flux capacity, twice as much base dissipation but unfortunately 1.4 base flux/dmg as opposed to the Onslaught's 1.0. On top of that, I would say the Onslaught has less spare OP to dump into dissipation than the Conquest.
If I had to put my money on one of those 2 ships in a duel between 2 highly and evenly skilled players, I'd bet on the Conquest.
I also think the Conquest beats a Paragon because the only weapons the Paragon can use that would reach the Conquest are the High Intensity Laser and the Tachyon Lance, both of these choices are rather useless if that's all you can hit your target with, dealing mediocre soft flux damage to a Conquest with extremely good dissipation.