Assuming that you are able to achieve a high enough rank for each term to actually get visitors on them.
Typically phrases are much easier to rank for than single words. You may find that you achieve a SERP position of, say, 12 for the term "widget review", which brings you quite a bit of traffic.... but a position of 350 for the term "widget", and since no searcher drills down that far in the serps you don't actually receive traffic on that term.
The question then is also whether those visitors actually have any value for you.
"widget review" - presumably this is what your page actually offers - reviews of widgets. If your visitors arrive on that term, you are happy because they want what you got, and they are happy because you got what they want. This is a high value visitor.
"widget" - maybe they want widget reviews... probably more likely they want to buy widgets, or find out what a widget is, or find a copy of the instructions that didn't come in the box with the widget they already own, or they're looking for an obscure enough photo to be used in a silly competition on some webmaster site. This is a moderate value visitor.
"reviews" - the odds that the reviews they want are of widgets? Narrow to none. This is a low-to-no value visitor.
So.. if the ranking scenario above occurs.. what do you do?
1. LEAVE YOUR PAGE ALONE!
It ain't broke - don't start trying to fix it. If it's working for you on the 'widget review term', you don't want to lose that traffic.
2 MAKE ANOTHER PAGE TO TARGET THE TERM WIDGET.
It's going to be a longer, harder road to rank on this term. You'll be glad you didn't throw away your 'widget review' traffic while you're working on this. More to the point, even if you achieve the rank you would like on this term, you may still be disappointed with the conversion/retention (whatever metric is relevant to you) rate on these visitors, simply because the term is untargeted.