I doubt there are many people on this board with experience with these two. You may be better off throwing it up on the Jim Saubier's board. The small bore guys hang out there, but mostly bolt action discussion.
I have built 4 uppers in Tactical 20 and about a dozen in 17 Rem. Both will shoot 1/4" groups at 100 yards. The 17 seems a bit more finicky than the 20T and some bullets don't shoot very well in the 17, while others pile them into one hole. The standard 1:10 twist for the 17 seems to work best with the heavier bullets in the 30gr range. The best for me was always the 30gr Starke, but they are out of business. So now I shoot the 28gr Calhoon. For 20 and 25 grain bullets, I would lean the twist out to 1:12 in the 17.
The Tactical 20 shoots everyting pretty well, and the Hornady 33gr V-Max will stack them into one hole at 100 yards. Tightest groups so far measured .153" at 100yds fired by a gun writer for Small Caliber news. The published load data on the 20T uses mostly powders that are too fast burning. Using slower powders seems to work better. With Berger introducing a bunch of new 20 caliber bullets in the Fall, the 20T should get a jump start.
If the choice were mine, I would go with the Tactical 20. Same speed as the 17 rem, but you're pushing a little bigger bullet out there. I have only used 24" stainless barrels on all the small bore uppers that I have built.
Turn time to build one in either caliber would be about 10 days.
Tony Rumore
Tromix Corp
www.tromix.com