You may want to consider a PSR-E400 series keyboard. They're still lightweight and not very expensive, but they do tons more than the E300 series. They typically have more sounds and styles, and starting with the E453 on up, can load up to 10 additional onboard styles (the E433 and E443 can load up to 5 additional styles). Starting with the E433 on up, they have 32 registrations -- arranged in 8 banks of 4 registrations, whereas the E300 series -- at least up to the E373 -- has only nine. I just checked the manual for the E383, and it is slightly better with ten registrations.
Additionally, the E400 series keyboards have a built-in mini-synthesizer that allows you to change the filter and envelope generator settings of most sounds, allowing you to create entirely new sounds, and these settings can be saved in a registration. These settings can be changed for the main and dual voices (the two right-hand layered voices) independently. However, they cannot be adjusted for the left-hand split voice. Also, many of these functions are accessible through live-control knobs.
Finally, the E473 is the first model with with dedicated buttons on the panel for octave and transpose -- it looks like you have to use a shift key for transpose, but at least you don't have to menu-dive to get to it!
It is pretty well documented that the key feel of these keyboards peaked with the E433, and that the later models, while not bad, do not seem as crisp as the E433. And of course, the E400 series keyboards are more expensive than the E300 -- but you get a lot more features.