A lot of people got burned by Castle with their earlier versions of ESC which were notorious for having a weak BEC, folks were getting brown outs and had to buy glitch busters and UBEC's to fix the design flaw. The newer Castle ESC's have fixed their BEC issue and provide a really good quality ESC today.

Not sure if Castle offers a "trade-up program" but HobbyWing will give deep discounts which have varied anywhere from 25%-60% (on the trades I've made) depending on the trade/age of an older/damaged ESC for a new one. That has kept me loyal with HobbyWing plus their quality has lasted longer without problems for me more than any other brand of ESC I've owned over the past 10+ years.

Anyway, you can't go wrong with HobbyWing or Castle, both are solid brands.