Linton,
An imperfect situation for sure.
Lots of valid points made by folks already.
The main points to bear in mind are:
1. Disregarding an RA is usually inadvisable. [for those of you reaching for the keyboard and wanting to get stuck into me for not saying ALWAYS, just don't. Use the search facility, read related topics on which I have posted and don't bother replying unless you have something new to say, because its already been done to death]. There are situations where it may be safer to not follow the RA. However, it is ALWAYS a bad idea to manoeuvre opposite to an RA. Hence if you can reasonably foresee a scenario which might cause that, you are better off being in TA only mode, because:
2. TCAS scenarios get more complicated and more critical when one or both aircraft in the encounter have inhibitions (re: possible RAs). E.G. if you are at low level, or a poorly performing light aircraft. Sounds like a factor for your operation. Specifically, being in TA only mode can provide the other aircraft with more options for a succesfull uncoordinated RA.
Linton, I've spent a lot of time thinking about TCAS (it was a project at a previous job for me). As I recall, a while back you passed through an institution at which I was an instructor. If you want to talk TCAS drop me a PM and we'll have a chat.
Regards,
pb