sipdroid only supports ulaw, which doesn't work too well.  Although I like their interface better, it seems like they should support speex over ulaw.  SipAgent supports speex and has generally better incoming call quality, but with a crappier interface.  Also, instead of using sipgate, you can get a free number from ipkall as well.  I hooked up the ipkall number to google voice so that I can send/receive calls from the same android device.