I havent owned either, but have used both of them from time to time.
The silvia is a much better long term prospect, as you realize. It also copes fine with 2 to 4 regular milk based drinks.
However, if budget allows you to upgrade in the next year to an HX and decent grinder then theres no contest. If it were me, id us the SB till she died and save the cash you can till you upgrade.
The silvia will cope with most of your current usage but the HX will do it better and allow for increased volume in the future (when the news spreads) ;)