The four parts of (the standard) 4LLL are:

1. EO (edge orientation)

2. CO (corner orientation)

3. CP (corner permutation)

4. EP (edge permutation)

COLL lets you combine #2 and #3 into one step, leading to:

1. EO

2+3. CO+CP (COLL)

4. EP

But from here, combining 1+2+3 leads to OLLCP, which is a gigantic alg set with 300-ish algs, while combining 2+3+4 leads to ZBLL, which is an even larger alg set with almost 500 algs. There's no easy progression from this 3-look system to a 2-look system.

On the other hand, PLL lets you combine #3 and #4 into one step, leading to:

1. EO

2. CO

3+4. CP+EP (PLL)

Progressing from here to a 2-look system is pretty easy in comparison: just combine steps #1 and #2 for full OLL ("only" 57 algs).

As 3-look systems, learning either one of COLL and PLL will have roughly the same benefit over 4LLL; the real difference is in how you can progress from there.

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Now, there's an alternative last layer system that might have some potential: CLL/ELL. (People dismissed this in the past, but these days people have generated better ELL algs.) It's a 2-look system, where you first solve the corners (CLL, 42 algs) and then the edges (ELL, 29-ish algs). If you know CMLL, most of them can be used as CLL algs too, so it's mostly only ELL that you have to learn.

Here's a list of all the algs you'd need for CLL/ELL:

https://docs.google.com/spreadsheets/d/1hNa6Fr8q7b_O2_Iji6S4G0zmSdGO8xihQxar1qHTeNw/edit