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Thankfully, things have moved on the last ten years and big improvements have been made to the hammer action keyboards and the sound quality they use, particularly in their piano sounds. As far as I know, the 300SX and the 700SX were made at the same time. The 700SX is considered to be [one of] Roland's flagship models. The 300SX has most of the same features, however, in order to make it as light and compact as it is, the 300SX keyboard action will not be as high quality as the 700SX, again because the 300SX was designed for portability and gigging being only 15kg. It's bigger brother weighs around 24kg.
The good thing about the 300SX is that it does use some form of hammer action keyboard, allbeit the "compact progressive hammer action". This is very much a simplified version of the 700SX, cutting out flash drives, more rhythms, tones, voices, etc. If you are just looking for a keyboard, nothing too classy, then this would be the one for you. If you have a more discerning taste for keyboard action and recording/performing, then the 700SX may be better.
Bear in mind however, that neither of these keyboards have their own speakers which means that you will need some form of external amplification. I know that Roland use the HP for home use as they have their own built in speakers. I've found always this to be a downside to the RD stage piano range if choosing it as a home keyboard.
You can see the specifications from each keyboard on the Roland UK website:
Roland 300SX: http://www.roland.co.uk/digp_room_catsp ... id=RD300SX
Roland 700SX: http://www.roland.co.uk/digp_room_catsp ... id=RD700SX
Hope this helps!