This is a theme that often surfaces in bridge discussion. I thought it would be useful to list the factors that would influence me to open 1NT or 1M, once and for all. At least this will allow me to simply point interested parties to this article!
Why would anyone want to open 1NT when 1M neatly describes the main feature of the hand, the 5-card major suit, which is also a very live strain possibility? In a standard system (featuring 15-17 NT openings and a forcing NT response to the 1M opening), the main reason is that you will have a difficult time describing your high-card power if you open 1M. After 1M-1NT, new suits can be as weak as 12 HCP, and the 1M-1NT-2NT shows 18-19. Rebidding your majors with 5332 is a big no-no (partner will be misled as to both your strength and your suit length, expecting 6 cards there, if you do this!). The solution is to temporize with the cheapest natural new-suit bid. It is a bad kind of solution; no one feels comfortable rebidding Two Clubs in this auction with KJ1042 AQ5 K10 Q72. But it is your best bet if you decided to open One Spade and heard One Notrump in response.
On the other hand (bridge always has another hand), if partner does not reply to your 1M opening bid with 1NT, you are probably fine. If he raises you, you are certainly ahead of the NT openers (this is the worst case scenario for them). If he bids something else at the 2-level, you can rebid 2NT without misleading partner as to your strength.
This is not an easy situation, and if there were a perfect solution, it would be already widespread. Always opening 1NT will miss many desirable 5-3 fits. Never opening 1NT will lead to awkward auctions and missed games with the points divided 15-9 and the like. The best answer lies somewhere in the middle, but how to identify that middle? This is what this article is for.
We are assuming here that the question is whether to open 1NT with a 5332 hand containing a 5-card major, by the way. Devotees of the 1NT opening will do it with 5422 or even stranger distributions, but here we are not that creative (these hands will often have a comfortable rebid, eliminating the main issue with the 1M option).
So, here we go, with a list of questions which you should ask before deciding. Remember, none of this criteria should be taken in isolation. If a hand fails a minor criterion but is overall fine for opening 1NT, do it.