Most of those are very similiar (monker, pio, gto +), of course there are difference in functionality/interface and so on. But you got to decide by yourself which one suits you the best.
Start watching videos(tutorial or pro videos) about them and get idea how it look/works.
And you are right, there are many other tools which helps you playing ( starshelper, hand2note.....).

What games you playing? MTT? NLHE?

playing around nl50 nlhe cash games, mostly the fast forward format
Okay, I tend to get monkersolver because it can solve everything, especially multi way pots
pio edge seems a bit expensive and can do less than monkersolver

Tools like monker are less of science and more of art when using a high level of abstraction.

Pio and GTO+ are actual equilibrium solvers but pio can also have a degree of abstraction (just more precise). GTO+ solves down to 0 exploitability while Pios current algorithm will not in most scenarios. Pios interface is much more intuitive and the funcrionality is built for pros.

If you give it a year or two there will likely be more conpetition with better tools at a lower price - as woth all tech.

All solvers use some form of abstraction to get the results. Whether it be a subset of flops or bucketing groups of hands with similar EV to reduce computational power required to reach the end result.

Monker uses buckets to acheive its ranges. You can set the level of abstraction. If you limit it down it will not be super accurate but it can give you an idea of what its trying to achieve.

Pio uses flop subsets of the game to acheive its results. Though you could use the full game (1755 subsets) to run the preflop solutions but then you would also be limited by the bet sizes you choose.

All the tools have restrictions. Preflop ranges are available and its probably cheaper to find someone to by them from. Most of the work about understanding exploits will be done with node locking. After playing with all the tools over the years id say just pick one within your budget and that does what you want.

The tools that are going to give you the best information are going to be the postflop tools. Pio, Simple Postflop, GTO+, etc..

I actually wouldnt even reccomend preflop tools if you can find preflop ranges to understand what optimal looks like because in all honesty you should be using your database to find what hands are profitable in your pool. Profitable ranges, especially RFI, change from pool to pool.

Most of your money will come from exploits and/or from opponent mistakes postflop.

EDIT: I think a good personal poker coach is more valuable than a pre-flop solver unless you are playing high enough stakes and even then its debatable.

EDIT2: Youd probably be even better off finding a coach thats done the solver work and shortcut yourself the time.

NOTE: this is all opinion. Dont take anything i say too serious.

"approximately same as with Pio, but you will need more RAM to do the heaviest solves. To solve the 4-handed pre flop game (CO v BTN v SB v BB) with decent accuracy, You'll want a 240 GB RAM setup, same as for solving pre flop HU play with Pio, using a big flop set.

To solve for 6-handed ranges with good accuracy, you will need at least 500GB RAM."