Wow, 76 versus 61, and the 76 is significantly lighter.. go figure.
Wouldn't a lighter yet larger keyboard suggest a compromise in action?
Why?? The weight could have been saved anywhere from the electronics to the chassis or case.
These arranger keyboards have been very good instruments for over 20 years, which means by the very nature of development, improvements are going to incremental rather than groundbreaking, model to model. Slightly different features, maybe slightly better sound etc. Over a few model upgrades these may add up but whether or not they are “significant” is very much in the eye of the user. Your musician friend obviously values the ensemble feature, but then it may not be of value to someone else. Maybe the weight reduction is more valuable if they have a bad back! And as for how much money they can spend to get what they want???…..their choice!
Fred answered your question well, you can stay within your budget, get 3 rh voices, built in speakers and play midis on an SX900. But of course there might be some conversion work, that is always a possibility when changing models, especially when jumping several models.
As far as getting expert advice, there are many comments on this forum on both Genos and SX range of keyboards, and many evaluation videos on both on utube. But as you have been playing an arranger for many years, you might be hard pressed to find anyone in a music store more qualified to asses a new arranger keyboard than yourself.
Mike