I went through this same discussion in my head a week ago. I decided that since I was already splurging on this new tool, I would avoid the the buyers remorse boomerang I always get when I semi cheap out. That is when I get the not quite the largest size to save a bit, only to discover a few months later that I could really use the extra capacity...So I went for the TS75. It is the best tool I ever bought.
It was my first Festool and I was dubious of the hype so I bought it standalone, and duct taped my shop vac to the collection port. It worked so well, exactly as advertised (how often does that really happen?) that I went back to the shop and bought the RO150 and CT22. My wife, who is normally fairly critical of these extravagances saw the dust collection capability and said "that is so cool"...so Christmas came early this year...