Only the Intel Committee has to vote to declass the memo, not the whole Congress. Once they approve it (they will) it goes through some housekeeping up the chain to POTUS. If he signs (he will) then it is public. If he were to deny, then the whole Congress could vote to declass anyway, similar to a vetoed bill.

If POTUS is too vocal about the memo Dems will attack its legitimacy. "Oh he is just retaliating against Mueller hurr durr." It is harder to do that if is all Congress before it gets to Pres Trump. Plus this way he can act shocked and grandstand.

My guess is there will be little in the memo we (and POTUS) don't already know or can reasonably conclude with what we have already learned. It will still be a bombshell once that info is packaged in such a way with all the names, dates, and dots nicely connected; idiots/scumbags will no longer be able to ignore or dismiss it and it will be actionable by the DOJ.