I'm in a similar situation, except I'm the other half. I've been in the UK two years under HSMP and just got my FLR (Tier 1), and engaged. Busy summer.
We're not planning on getting married till Dec 2010 so for me it doesn't make sense to switch to a Spouse visa, as it'll be faster and cheaper to get ILR/citizenship on my current track. I'd look carefully at how much each visa costs and what the total cost is on each option, which'll depend on the timing of when you get married vis a vis her Tier 1 path.
Another thing to consider is the impact on any current or future job hunting, as employers may prefer someone with ILR so someone on a fixed-term visa.
Of course, I'm also the fiercely independent type who's always said I wanted to earn ILR and/or citizenship on my own (not very romantic, I know), so that's been a factor for me as well. I'm proud (and lucky) I've been able to.
Best of luck!