Steve, after much thought, I have desided 6 of one and half dozen of another.....the best advise I can give is let your ears be the judge. You should try both systems at once and listen to both. If you can't use your piano on these tests, bring a small cd player with piano music. Both systems have pros and cons, but it just might be splitting hairs. If you get a small amp,they are often designed for guitars and distort easily, which is good for guitars, but not for new piano students. Of course a cheap monitor system could be prone to distortion also. Yamaha and Roland both make monitor systems and I would feel confortable using them as they should be designed to handle the load of a keyboard. An amplifier with low distortion is also a good choice. BTW 80 watts in a bedroom is LOUD! I have a 50 watt amp that I have to turn way down as it can scare the dogs down the block
It uses a 15 inch speaker to handle the lows generated by my paino and organ. A monitor system generaly uses tiny speakers (in comparison) for the mains with a small sub handling the lows quite nicely. This would be good in a bedroom. Bottom line, either system should be good as long as you get a low distortion model (if you want distortion, pay $20 for a distortion stompbox and keep the amp clean). For $300 you should be able to get a good amp or monitor system with change to spare. If your daughter ever wants to play outside the bedroom, school programs,church, etc. go with the 80 watt amp as her amp will have to keep up with the other musicians, as guitarist are notorius for having 100 watt amps cranked all the way up. Be carefull of amps with small speakers as the low end may distort at higher volumes. A two way system, large woofer and a tweeter or midrange horn should handle the large range the piano has. Let your ears be your guide. Good luck!