Which amp is right for the job?

Go with the loudest you can possibly get. You can always turn a 10,000 watt PA down, but not a small amp up once it's maxed out. What you want - with an acoustic instrument in particular - is enough headroom to play clean. When things are running on max, you tend to get nasty distortion and other unpleasant issues.

A regular guitar amp is not designed for acoustic guitars. An acoustic guitar amp has other types of speakers with a broader frequency range and they are designed to play clean. Regular electric guitar amps are designed to work best in the mid-range frequencies.

My guess is that the piano amp will be much closer to your sound preferences. However, 180 watts is very, very modest outdoors. Even a 1000 watt+ PA system can be too modest once it is outdoors. The open air basically sucks up volume faster than a speeding bullet. And then you have all the other sounds present. If the audience is dead quiet, then you may be heard with the amp...

Another thing, do make sure you have a quality EQ available to combat feedback and/or sit well away from the amp.

60W should be plenty, seeing as to how a 26W amp can easily overpower a 300-seat auditorium.

If you have the chance try out the 60W amp beforehand.

You should get plenty of sound out of the 60W.