a lot depends on soil conditions and how the t post stand up, I use the heaver T posts,
I have been building my newer fences out of both,
one wood post and 4 Tee posts, one wood, 4 tee,
the old fence is all most 100% wood, a tee post about ever 20 or 30 to drain or ground out lighting strikes, save having to go and replace 10 or so splintered posts after a storm, and have not had any cattle hurt since adding the steel post to the fence.
yes I agree the new wood posts seem very weak, and snap easily,
I use 4-5" wood posts, normally the deal is (at least last year) the cost difference was only 50 cents or so, the work of putting them in is part of my consideration, (they seem a little harder ever year now).
we have over 10 miles of fence to keep up, I need to rebuild a mile and half this summer some time, two fences that are seldom used, and about 50% of the posts are in need of replacement,
and I have a bunch of corners to build or rebuild as well,
I am considering trying some different on the corners, and that is to about 100 to 150 feet out from the corner start to bring the wire down to near the base of the post, so the stretched wire has less leverage to work the corners back up out of the ground, and then use a "short" wire to fill in the fence where they should be, in that 150 or so feet, in this sand in 10 to 15 years the corners start to lift out,
I guess I could bolt on some steel wings to the base of the posts so they would help anchor them, and still use the post driver, (post driver new), in the past they were all hand dug holes, and most were over 3 feet deep usually use 8" round posts on the corners,