Ranger REG- you have read the d20 Mecha rules?????
How do you know that they are more anime than armagedon's rules? I have the Mecha Compendium (from DP9) which use the rules from d20 Mecha (from GoO), and inside you'll find d20 stats for the Mecha of Gear Krieg, Heavy Gear, Jovian Chronicles and many new setting.
The author of d20 Mecha (David Pulver) is the author of GURPS Mecha, and while I did not like this book (while I liked GURPS Technomancer from the same author), because it is more focused on GURPS rules than typical GURPS sourcebook, the system, as the system from BESM (which appeared first in Cool Robot Big Starship, still from the same author), is generic and meant to do any mecha style.
Also, it was mentionned that Mongoose had access to the d20 Mecha rules and will try to make their Mecha as compatible as possible with Mecha produced through d20 Mecha, and if they did not use the rules, it was because setting specific elements changed the cost of several features, while GoO book is effect based (i.e. you choose the stats, then you'll have a cost, while many construction system let you choose the elements of your construction, then you figures the effect, there is chances that it's why Mongoose use a different system).
I don't know BattleTech, but based on this if it's a near futur setting, Armagedon will certainly work, and provide many rules for character war tactics, etc... But if it is based around any other time period, depending on how much the original poster wants to spend (d20 Mecha is at $19.95 for 80 B&W pages softcover while Armagedon will be Harcover Fullcolor high page count and around $40 IIRC), to perhaps choose d20 Mecha. I know that I'll wait for review of both before taking any of those.