Just FYI, Yahoo also does not "delete to trash" with the Blackberry BIS, so it's pretty much the same as GMail.
I use both -- I have Yahoo configured via BIS to get pushed directly to the Blackberry where reading long emails is better (due to the fonts), like my subscriptions to DailyLit (reading books via email). I don't use it for normal emails, mainly just subscriptions and such. It also allows me to hand out an email that I know will go right to my BB device (like if you set up AMBER alerts, hint hint.
)
I have GMail configured via BIS but *not* pushed to my device, so I can send emails from the device using the builtin tools (i.e. when you want to send a webpage link or something). I use the java GMail applet you can download for actually reading and replying to my normal GMail, because it offers full Sent/Trash/Spam support - it's like using the normail webUI.
Sometimes there's just not one tool or method to rule them all, you kinda have to patch together a situation that works for your needs.