IMO the issue is a little more complicated than folks have said so far.
If you make only 11x14 prints, there's very little visible difference between M8 & 9. The difference in IQ is increasingly visible in larger prints; that's where you'd notice (as Roger said) that 'the way the M9 renders texture and tonality is a great deal better.' The lens you use will make more difference than the sensor sizes at this relatively small print size (Brian's wedding picture is an example of a high flare, low-resolution lens). You'd be more likely to notice the IQ difference –
1. If you crop a lot (i.e., too much);
2. If the file is noisy from underexposing or from shooting at too high an ISO (which raises the lens issue);
3. If you get tired of making 11x14s – which are now small prints from the standpoint of exhibiting them – & decide to print larger.
Barring these circumstances, an M8 (M8u, M8.2) will suit you well for the size of prints you make.
BUT: The M8 is pretty bad re: noise at higher ISOs, & M9 is a little better (less magnification/res-ing up, & apparently some NR at high ISOs built into the firmware). So if you do much available light photography, you'd benefit from the M8 + Lux combination, & a bit more from M9 + Lux.
My personal solution for M8u was: (a) 28 Summicron (35mm equivalent); (b) vintage 35 Summicron for daytime (ver 1 or ver 4, I use 1); (c) 35 CV Nokton 1.2 for low light; (d) 50 pre-aspherical Summilux. (When the new 35mm Summilux – with reduced focus shift – is available, I'll start using it on M9.)
If you buy an M8, do look for a used M8.2 (or M8u). The viewfinder is more useful and the newer shutter, while lacking 1/8000, has lower tension & should be more durable.
So my 2 cents of advice would be: just start with M8 & the Summicron from your M6 (I hope it's 35mm), & see if you need anything more than that. Find a good used M8.2 (or u), & you won't get stuck for a great loss if you later want M9. You mentioned you're parsimonious, & that would be an outlay of only $2.5K or so.
Kirk
PS, did anyone direct your to
www.reidreviews.com ? You'll find extensive reviews of all the current lens options & 1 or 2 vintage ones.