ive found they usually go hand in hand. in the commercial world, install is almost always easier to keep the guys busy. we sell whole landscape installs, and have a good sales person. we usually do 2-3 installs a year (most are multiyear/phased). i have one crew that does the basic service and i coordinate the install. i will also help with the hard to troubleshoot service calls.
Lately, it seems we are trending more to have the department manager (myself, construction, and design) function as project managers. Unless there is a lot of hardscape on a project, my crews and i will take a project from groundbreaking to ribbon cutting with minimal assistance. This makes scheduling much easier.
Given all that, if i could keep busy with install, i would give up most of the service we currently do. almost all of it is "we will call you, dont do any work without a work order" type of accounts. Its almost always a 2 visit service call to get any work done.
but, i am happy to not have to deal with 2500 customers anymore.