I'm firmly in the "mild steel" camp, can't see spending the extra money on stainless. Turns out it was primarily the lead in gasoline that ate exhaust systems before (actually other additives that went along with the lead to reduce lead fouling). Of course they don't salt the roads around here, so your results may be different if you drive on salted roads.
I bought a mild steel repro system from Moss around 1985 or so, from the headpipe back. Around 2002 I had a local muffler shop replace the main muffler, as all the fiberglass packing had fallen out (making it basically a straight pipe) but the shell of the muffler was still intact. And the rest of the system was still in good condition (except no fiberglass in the resonator) when that car got wrecked in 2005.
That experience spoiled me; I'll never try to install my own again. Cost was about the same, but the entire job took less than 30 minutes from start to finish and I spent the entire time sipping a cold drink and reading the newspaper. Best of all, I didn't stagger into the house afterwards, looking like a coal miner after a cave-in!
This time around, I just drove the 'new' TR3 back to the same shop, and had them install Cherry Bombs for both muffler and resonator. (The Cherry Bomb mufflers use the same straight-through glasspack construction as the original mufflers, but are round instead of oval and seamless instead of having rolled seams.) Sound great to me (although I do still very occasionally set off car alarms).