The main difference would be that Onslaught is such a lower tier set that the itemlevel and the attribute stats on the pieces and 2piece setbonuses would be drastically lower, along with the armor of each piece.
The offensive firepower is close to being the same, but with the higher tier mix allowing for a slightly better spread of magicpower when you get Int overcapped (main stats get softcapped at 1050 int/str/balistic/willpower) and instead of going pass the softcap and losing some dimininish return value on getting more mainstat, you instead can go to get +magicpower and continue to gain 1:1 bonuddamage as if it was int. Just without getting the secondary bonus from your mainstat such as strikethrough from int, you dont get that from magicpower.
I ran onslaught mix for aoe build for the longest time, the proc can overwrite itself and provide you with close to perma uptime if you are hitting enough enemies.
But in the case of wanting to respec from time to time into Singletarget build the onslaught set were drastically worse for this build as the timestamp rotation is not great at giving BoostV proc chances and you are not hitting enough targets before the burst damage to make it worth while in this build.
Running Sove + LOTD is a very nice setup to run in both ST/AoE specs and it also provides you with the higher armor values on gear, higher stat values potentially opening up swapping out your int tactic and letting you run an other tactic if you should chose to do so.
Running sove+triump+lotd gives you a good portion of wounds also when you finish the ring grind. hope this helps.
There might be a tip or tick about BW over here also if you are interested:
https://docs.google.com/document/d/13Bf ... a0n12r60i0