Regarding emulation mode -- what this does is change the User Agent sent to a webserver; scripts and such can use this to deliver custom content (the classic "this shows up in IE but is missing in Firefox!" threads).
Blackberry:
"BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100"
Microsoft IE:
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0) BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100"
Netscape:
"Mozilla/4.8 [en] (Windows NT 5.0; U) BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100"
MS Pocket IE:
"Mozilla/2.0 (compatible; MSIE 3.02; Windows CE; PPC; 240x320) BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100"
Openwave Gateway:
"BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100 UP.Browser/5.0.3.3 UP.Link/5.1.2.1"
Openwave (WAP):
"BlackBerry8100/4.2.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/100 UP.Browser/5.0.3.3"
So, your "optimum" setting really depends on your website -- if they have Blackberry specific content versus wanting to see it like IE does on your desktop and so forth. Some sites will look better in one mode and some in the other.
edit: decided to save this info >>
here<< for easy bookmarking