I would pay attention to any mike's polar pattern, and buy what suits your regular stage setup.

Most keyboard players do not have monitors directly in front of them (damn keyboard gets in the way!), but unfortunately, most cardioid mikes null point (the point where the most feedback rejection will come from) is wherever the mike is directly pointing away from. Which is always pretty much directly forward. Where YOUR monitor is NOT...

Hypercardioids and supercardioids reject more from about 45º to the side of the mike rather than directly in front, making them, IMO, far better at feedback rejection for systems where the monitors are at the sides...

I also feel that head-worn mikes need FAR more care with vocal compression than regular stand mikes. It is VERY difficult, if you can't ride the mike, to go from warm and intimate to bright and full throated without a good compressor. Especially as your arranger isn't going to ride those dynamic changes much at all (ever notice how the arranger doesn't really LISTEN to you? ), so you need a very even sound, no matter your technique...

I have heard SO many singers using head-worn's that are either deafening you or inaudible, often on the same song. If your voice is already VERY even, with little dynamic range, sure, go for it. But I would factor in the cost of a really good vocal compressor (the ones in arrangers are pretty dire, IMO) to the mike if you are not...