also..if you check out "Overview of testplans" in the help file, there is a pretty good explanation of some of the additional things that you can do with a plan that you can't do with a suite.
My reasons for using plans are:
1 - Can run different parts of a plan via a query that selects certain parts of a larger plan. (i.e. only run smoke tests). If you want to only run part of a suite you have to manually comment out the lines of the suite you don't want executed.
2 - You can call plans and the queries that you like from the commandline. With suites we'd have to have several suite files with different permutations of how we want to run a test and then call the appropriate one from the commandline.
3 - TestPlanEnter and TestPlan Exit functions. These recovery system functions make setting up for running a group of tests nice.
4 - You can load different option sets from the plan so that different parts of the plan can be run with different options. Can't do this from a suite.
5 - You can call functions and specific testcases from a testplan whereas a suite can only call a script or another suite.