I've owned both and "prefer " the 911 - why : First 911 was 86 Targa that had the guides and all 3.2 work done for $7k by previous owner. I updated some suspension peices and it was tight.
The 951 was an 87 with rod bearings and H20 pump done by PO. I drove both at the track - both around the neighborhood for better part of year. commuting was easy with 951 - good A/C, comfortable driving postion, easy clutch, all power opts. It spun up real quick, but too quick for my liking (since then I also have a 930 so I know about boost). The 911 was better for cruising, and spirit, and track balance IMHO. I sold the 951 just in time fo the next owner to have the head gasket go (ouch
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The 911 cost me $13 in 1998 and the 951 was $9k in 2001