First, let me state that both balls work very well for me. I consider these 2 balls excellent choices for a large range of styles and conditions.
What's the difference? Not much. When you can use one, you can use the other. I think only the most discerning bowlers will value the difference. The Marvel Pearl seems to be a touch smoother and slower to react. The SoniQ is surprisingly strong for R2S pearl. It is quite rolly and screams controllable power.
Do you need them both? I'd say no, but if you like to experiment with layout and surface you could make them a good 1-2 combo. No matter what you do, they really won't be that much different.
My Marvel Pearl is at 4000 abralon drilled 55x4x40 (pin up) and SoniQ is box finish at 45x4x70(pin down)
The shine on the SoniQ was suprisingly smooth and not very over/under. I think the layout helps. That pin down, early roll setup is my bread and butter --- particularly on house china.
-- and a side note -- SonIQ vs HyRoad. Soniq is 5-7 feet earlier and WAY less over/under. HyRoad is way hotter off spot.