Dennis,
if you already have the license for IR, then I would definitely go with it.
Here the reasons from the top of my head why:
1. Price
If you are planning on doing routing via SCCS, you will need to have license for each user. If you are using IR, and are planning on not using SCCS reporting, then you do not need to buy license for Symposium.
2. Reporting
Symposium reporting is very accurate but it is really really primitive. We are using CrystalReports here and it is just a string of numbers. To me Symposium and PBX are of the same level of importance, so you do not want people to start meddling with it. With Genesys, you would use CME to add/delete agents, Strategy Builder to add/modify strategies and if you really mess up and crash the whole system, your basic call center operations would still be functioning. On top of that, you can just import your last CME backup and you would be able to continue as nothing has happened (well, in MOST cases).
If you mess up with Symposium, you better have your PBX people standing by.
3. Scripting maintenance.
With symposium all the routing is written in script, plus you have a rather limited (if any) access to the current statistics to make decision for your routing. You would also need to pay your PBX support company to change it for you.
With IR, your strategies look like a visio diagrams all of the components are drag
drop, plus you can play with it as much as you want (needless to say, I would not suggest doing it on the strategy that is currently running)
The really big minus for using Genesys for routing is that it is not as stable as Symposium. It gives you flexibility, but you would be losing on on stability. I would say one call per 10,000 dropped to default using Genesys is pretty common, while in Symposium this is more like 1 in 100,000...if any at all.
If you need some strategies just email me directly and tell me what kind of strategies you would like. I don't mind helping you with it.
Vic