Well, Tirexes are faster. However, I also like Giants better than Tirexes. However, if you go on map thieving spree, you may not find them on Tekron (Montero map is not available on other islands), while it's guaranteed to find Tirexes on Reha.
That certainty helps a little. Otherwise, Giants are great - 900 HP, no vulnerabilities (Tirexes have -10% fire), great damage, less leadership requirements, better base defense than Tirexes (important if you use them as tanks).
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