The grammar is okay with both, but neither is very natural. I would suggest: He always resorts to personal attacks instead of presenting a coherent argument. If you resort to personal attacks instead of presenting a coherent argument, you know you've already lost.

"Ad hominem" is a Latin term. For that reason, we always put it in italic font. This platform does not support italics, so you cannot do that here. Instead, I will place it between asterisks, and you can pretend that it is written in italics. The reason it is always in italics is that it is in a foreign language. It is Latin, not English. We put foreign words in italics. In Latin, it means "to (the) man (or 'person')". Your sentences are not correct because you use *ad hominem* as if it were a noun. It is not. It works as an adjective or adverb: "That is an *ad hominem* argument" (adjective) "He argued *ad hominem*" (adverb) Notice that you can replace *ad hominem* with the English translation and it still makes sense (as it should): "That is a to-the-person argument" "He argued to-the-person" With your sentences, this fails: "He resorts to to-the-person" (doesn't make sense) 1) In debate, he always resorts to *ad hominem* attacks devoid of solid arguments. 2) If you don't argue from the merits of your case and argue *ad hominem* instead, you know you have already lost.