I would play it safe and liaise with your partner/support to help with your upgrade and to recommend based on your findings which version to go with.
Saying that I am using SR7 at the moment which seems quite stable, unsure based on your applications though if you would still get problems I'm afraid.
I'll give support a call tomorrow, but I can't help thinking that they'll recommend the latest SR as this should be the most stable, with the least amount of bugs - But, SR9 caused some apps to crash when performing simple tasks and SR8 introduced a new bug with the calendar.
I might go for SR7 anyway as that seems to be fairly stable and I think used amongst a few people from what I have read. We've only left it this late, as I'm nervous about causing issues on our PROD server