I've tried both and am torn as to which is "better". They both have their strengths. Maxing your Pierce with Razortail is noticable. The draw of tgods, imo, is not the +3 LF.....that's fairly insignificant....it's the other mods....specifically....the lightening absorb and max lightening resist. The +str/vit is also nice....especially if you want to weapon switch to a spear with a higher str req. I like tgods better if I'm doing Worldstone runs. The lightening mods are very helpful versus Black Souls....and there tends to be a fair amount of lightening immunes in the WS. So....the choice kind of depends on the situation imo.
For cows, Razortail (or actually Goldwrap, but that's another story). For normal gameplay, tgods.
Cows has huge packs, where it's actually possible for your jav to keep piercing for a very long time. With 69% pierce, there's not much chance of piercing more than four times (15.6% chance of piercing the fifth time). Say your herd is 8 cows long, and you'll pierce 4 times without Razortail and 8 times with. That's a doubling of your damage. No way a couple of +skills can compete. And in cows, you don't really need that much res.
Different people different preferences. For me I will use ThunderGod. Adding tons of HPs, STR (which I can save for VIT), and superb Lightning Resistance. +Skills is just a nice bonus. All this extra for may be a few more LF.