MIRx is fantastic, but be aware that it is NOT a plug in. You can only use it on VSL instruments, beacause it resides in the VI (or VI Pro) player and is applied to VSL instruments pre effects bus. I believe that MIR and MIR 24 (which is the same as MIR but limited to 24 instruments) don't have this limitation. Check with VSL to be sure and demo them to make sure they work for you.
The suite is fantastic. I'd say it's comparable to the FX included in the Waves silver bundle that a classical composer would use, with the added bonus of having hundreds of presets for just about every classical instrument. The compressor and EQ presets make it so easy to get a great "life like" sound, which is really handy for me as I'm not a hard core audio engineer. (Before getting the suite I generally ended up doing more damage to my mixes with compressors & eq then helping them!) Also, the reverbs are excellent. Here's a link to some of the Suite Presets
I'd take up Paul on his offer to demo them both. And what you end up buying really depends on the style of music you write, what FX you already have, and how many instruments you generaly use. The MIR products are all about easily placing a group of instruments into a homogenous space with breath taking results (it takes "reverb" to a whole new level). The suite is really a more traditional FX bundle with the added bonus of almost all of the "tweeking" already done for you. Take your time with the demos and pick what's best for you and your music.
Best wishes,
Michael