If you're going off trail, are we looking at cross country travel, or a lot of bushwhacking through thorny scrub? If the latter, the very design of a poncho/cape with it's billowing sides will get caught on all sorts of branches and be a pain to wear, regardless of durability issues.
If you want the most bomber material, Dyneema X gridstop material (a la thru-hiker) should be near indestructable. But it's rather heavy at 4oz/sqyd. There's cuben fiber in the 1.2-1.5oz/sqyd with heavy mylar coating to help against punchter and even a 2.5oz/sqyd with extra spectra. But I'd say your 70d silnylon will probably hold up well and be the cheapest solution.