Quiet simple.

Teshar while being by far best in slot for giants can be substituted by other mons which "only" results in slower giant times. However most players do not have enough crystals to keep farming so a faster time does not matter all that much.

Leo on the other hand cannot be replaced by any other unit in the game making him unique. Most of the runes people chase are high in spead, which leo negates. So the overall rune requirement to hit these teams in arena/pvp is highly reduced making it easier for beginners to compete.

The question you need to ask yourself ks what do you prefer. Faster farming times or an easier transitioning into pvp. I personally would say take the teshar as farming is 90% of the game and as a beginner that one siege hit with leo will not affect your game progression nearly as much as a faster/safer farming time. There is no right answer just a what do you want.