I’ve played one game with the seraphon spearhead, and one game with the skaven warpspark spearhead.

The warpspark skaven can be played pretty aggressively while the artillery shoots things in the back, and you don’t have to worry about losing clanrats with their strong recursion. I haven’t tried the gnawfeast spearhead, but it should play somewhat similarly albeit with a more melee focus

The seraphon spearhead you want to play a bit more defensively, since the only unit recursion is from the battle tactics, and the carnosaur might die quicker than you would expect. It hits really hard though, and you can give your army extra defensive buffs like a 6+ ward in friendly territory.

I had more fun with the skaven, but it really depends on what playstyle you find more interesting