I have not used T715 so I can't comment on that one. Between the C510 and W705, C510 has better sound and build quality. Battery life is about the same. I always flash phones to the latest firmware for stability. There is one thing to consider. W705 has Wi-Fi. It comes in handy when I travel out of the country and need Internet access.
I see there is this "phone protection" thing, but it doesn't really seem to do much. Maybe there is just something off with my phone? But I was wondering if there is a way to password lock my phone. When I do the protection or locks, it doesn't really seem to lock the phone. I can still fully access everything. I want to be able to lock my phone and set it down and know that nobody can get past the first screen. Is that possible?
So I'm thinking of buying the C510 (for my parents) and I was wondering, what class of HSPA does the phone have? http://www.gsmarena.com/sony_ericsson_c510-2640.php <- not much help since it only says the phone has HSDPA,HSUPA with no listed max speed. As usual SE's site does not go into specifics like that.