I also work with both system.
If the system is only used for call center (no admin type user), I'll use Aspect.
If the system service both call center and admin users (almost half and half), I may still use Aspect but install another smaller PBX for admin.
The cost of maintanence will certainly be higher in Aspect (charging on per concurrent user basis). I'll not use the Aspect phone for admin user.
However, in a call center environment, Aspect I find by far more superior than M1/Symposium. Reporting in Symposium is just a joke. You cannot develop any good in-depth report from Symposium database. Creating your own complex report using Crystal in Symposium world, you will have a hard time to make the report scheduling work. Also, to administer the M1/Symposium, you need to have both M1/Symposium admin knowledge and still may not have a good handle of it. Not to mention you need to deacquire a set in order to make the button change.
Aspect by far is easier to administer, trouble shooting, provide a much better historical database structure to develop report, change call routing rule, trunk groups, etc almost instant. Plus it also offers a board platform for CTI, IVR, Multi-media intergration.