Why two spinners?
Two (or more) spinner games are rare in MAME. If you have a specific title that you must have that requires two spinners, then go for it. If it is just a matter of "going all out" then I recommend you skip it and save the money (and panel space).
And the same goes for eight buttons - if you have specific games in mind that require these, go for it. If not, don't add extra controls just because they are laying around or the panel has space for them.
I've learned that the experienced builders are always trying to minimize the controls on their panels. An uncluttered panel is usually more playable.
+1 on what Token says here.
Also, I feel like I say this all the time but it's rule #1 for me:
Decide what games you want to play on your arcade machine first...THEN build your CP layout around that. Why?
Because it keeps you honest, that's why. For example, as a fun little test, get a pencil and paper and literally sit down and write down all the "must have" games for your cab. Don't cheat and use the internet to look up games. You must try to think of all the games you want to play off the top of your head. (I bet it's 20-30 tops) Ok, now you can use the internet to look up other ideas for games you forgot about. Now I bet your list is somewhere around 100-150 tops. "But what about all those games I don't know about but might want to play someday?" Ok, well I bet even with those games added in, you are now around 200 games give or take. And I bet you will find that 2 spinners, a steering wheel, and a flight stick not necessary.
200 games is easy to manage in your FE and you will feel like you have built something that is more tailored to your games. If you go about it the other way around...building a CP that
could play ever sigle game made, I think you will find a CP that doesn't feel like
your control panel. A CP that is a jack-of-all and master of none, if you will.