After talking with a person in tech support and after much deliberation, I have decided to get the TX81Z. There are some things that are working in my favor:

1. Most of the listings on eBay for the TX81z are selling for a good amount more - if things don't work out I won't lose much money trying to resell it.

2. I found out that Manymidi.com has over 2000 GM type patches for the TX91Z that appear to be improvements of the stock presets that I could purchase for just $50.

3. The person in tech support told me that the result of what I wanted to do would be difficult to determine until I had the unit, and so it seems I might as well give it a go at $100.

We'll see what happens. Wish me luck.