I guess the most subjective part about this post is sound. Most keyboards that are on display in stores have really been messed with, mainly by kids who don't know a thing about arrangers but they sure love to push buttons.
Do a master reset or System Restoration on any keyboard you intend to test--this is important. Next, most keyboards do not sound good right out of the box. On Yamaha boards, go to the Mixing Panel and tab to the EQ page, then scroll down to User1/Edit and adjust the EQs. A good starting point for the 5-band graphic equalizer would be: 5, 4, 2, 4, 8. Then store the EQ settings in STORE1. Makes a big difference.
There are loads of options with fingering modes, including Full Keyboard, which I assume is the way you wish to go. Most manufacturers have this feature, and it usually requires at least three keys to be pressed before it will recognize the chord. Those keys can be pressed anywhere on the board. You can, however, adjust the split point to the extreme right side of the keyboard to make some very creative changes. I only know of one person that does this, and he always sounds great.
Finally, instead of listening to the keyboard through either onboard speakers or the store's ElCheapo PA system, take a good set of headphones with you and listen to the keyboard through them instead. This eliminates outside noises from kids pounding on instruments in the store and other distractions, thereby providing you with a truer picture of what the keyboard actually sounds like.
Hope this helps,
Gary
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Travlin' Easy