Grip: Light 2.4 vs All round 2.25

Usually a wider tire will provide more grip. Bigger contact patch. But many people ride a smaller, less aggressive tire on the rear as opposed to the front.

Just depends on riding conditions, riding style, type of bike, tire pressures, etc..

Really need more information to help you out.

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Well, it's just the combo I ask that I wonder about.
Wider tire increase grip. More aggressive tread/knobs increase grip. But if you don't want to have both (wide with aggressive tread), then wide only or aggressive tread only?
I'm 145 lbs riding xc and light trail. I observed some difference but not so sure about what I observe, thus this thread to seek other opinions.

Have you played with tire pressure on your current setup? What are you currently running?

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Well then,
Currently I run Wolfpack MTB Speed 2.25 at 18psi in the rear. Lets leave out Wolfpack MTB Cross 2.4 at 16psi front from the consideration. Rim has ~29mm internal width so it support both 2.25 to 2.4.
While front tire planted nicely, rear tire isn't as secure on sand/mud. Which is as expected because it's just low knobs 2.25 tire. From my past experience with Maxxis, This Wolfpack's compound grip really nicely on damp rock, as long as there is something to grip onto. But on loose terrain though...
So, for the upcoming set of tires once this one is worn down, I'd like to up rear tire grip a step.
But I'm not sure if I should up a tire size (MTB Speed 2.4) or up the tread knob height (MTB Race 2.25).

For me, the traction advantage of a wider tire exists because with more air volume I'm comfortable reducing pressure.
I'm not certain, but expect that with the same pressure from one tire to the next (narrow to wide), traction won't be much different.
But if you can lower pressure... then yeah, everything else being equal, you'll increase traction.
Hence my personal preference for wider (up to a point) tires.
=sParty

I think either way the larger volume tire provides more control. More control for me equals more confidence and more fun. So unless I was actually cross country racing or trying to keep up with the fast boys it would always be the larger tire.

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I personally like the wider tire up to a point. On the front a 2.4-2.6, out back a 2.3 to a 2.4 (depends on the tire) with a more aggressive tire up front with a less aggressive tire out back in a staggered sizing.

My bike can squeeze a 2.35-2.4 tire on it without rubbing on the back. Right now a 2.35 Michelin Wild Race'r sits out back and up front. . At 20f/25r psi, I get plenty of traction in most conditions, excluding sand and mud, but I don't ride in those conditions. Those conditions would require require a totally different tire.

I've found larger small-knob tires get very good traction on rocks/roots, but, for loose conditions, you might be better off with a more aggressive tire.

I ran a Rocket Ron 27.5x2.6" on the back this year and really liked it, but I don't know why Schwalbe doesn't make this tire in 2.35-2.4 which would be a really nice XC/Trail size rear tire, since it's not a "semi slick" and has big enough knobs for loose or soft-ish conditions.