One thing to check for is the possible drop in FPS in switching to ISBoxer. I used keyclone a couple years back 5 boxing on a core2duo rig with 8meg ram with little problem. This became a bigger problem when I switched to ISBoxer where I had to play around with screen resolution of each window to get 30fps just on the main (10fps on slaves). Of course a) no idea how sillynameboxer does window switching and what memory it uses b) if you have 43 cores and 5 terabites of ram, it' may not be a problem (but if you claim you actually have this, expect Sam to post saying he's got 54 cores...)

Also compare features and check videos; what can take an hour with Keyclone now takes me 30 seconds with ISBoxer. The features I love are:

a) super accurate mouse broadcasting
b) region specific mouse broadcasting (i.e. always mouse broadcast in a specific part of your screen)
c) Jamba incorporated within automatically if you have it installed
d) dual-boxing.com
e) cake
f) Driving my 2 cars at the same time. But lag's a bitch.