Is it worth becoming a Corporal to eventually become a Sergeant, or is it best to stay a Specialist and eventually become a Sergeant?

Corporal is an NCO. If your unit is willing to pin Corporal on you and you are prepared to handle the responsibility (with guidance) then you should go for it. As a general practice becoming a Cpl. is a tough transition and should come with it deffinative support from your COC until you gain the respect of your peers. It should already be evident that you are ready to step up.

The Corporal Rank gets a lot of crap for still being E4, however that's the wrong mentality! E4 is a pay grade. Corporal is a rank and NCO. Wear it, be proud, and stand out above your peers. Think of it like an internship to becoming SGT. You will be better for it.

Not every unit has CPL's. Some do, most do not. CPL is the optional junior tier of NCO. If you have a chance that you earn? Take it. Your soldiers will thank you later once you become a SGT.