For me, the main issue is size and whether or not bigger equals better. Does size really matter, or is there a point you can say “it's enough that I can tear the roof off that car/cathedral/sports arena/polar ice cap”?
Now I open the question to you: what's your dream mecha size threshold and why? I've diced the field into four categories for your reference below.
Powered Armor (<5m):Hard Suit, Battlemover (Bubblegum Crisis); Fuchikoma, Tachikoma (Ghost in the Shell); ATM-09 Scopedog ( Armored Trooper Votoms)
Pros: sleek, quick; easy to put on/power up; intuitive movement interface (natural body movement); with careful operation collateral damage can be minimal
Cons: can be crushed underfoot by anything from a larger mecha class; weapons limited to lower-powered hardware; possibly still vulnerable to conventional arms; for dudes, a swift kick in the nuts will probably still sting
Small (5m-10m):Aestivalis ( Nadesico); Orguss (Super Dimensional Century Orguss); AV-98 Ingram ( Patlabor); Walker Machine (Combat Mecha Xabungle)
Pros: relatively quick, maneuverable; strong enough to use a motorcycle as melee weapon (style points); wide choice of weaponry; ability of flight or hovering possible
Cons: still possibly vulnerable to high-powered conventional weaponry; likely loud and unstealthy; compared to powered armor training in operation likely to be more difficult
Medium (10m-25m):Mortar Headds ( Five Star Stories); VF-1 Valkyrie ( Super Dimensional Fortress Macross); RX-78-2 Gundam (Mobile Suit Gundam)
Pros: can jump buildings in single leap, kick tanks like soccer balls; options for transformation wide open; can tote battleship-grade weapons
Cons: don't even think about moving around unseen/unheard; learning curve to operate likely steep; chance for collateral damage high; where are you going to park this thing?
Large (25m<):Unit 01 (Neon Genesis Evangelion); Megadeus (The Big O); Getter 1 ( Getter Robo); Gunbuster (Aim for the Top! Gunbuster)
Pros: nigh invincible; can carry world-destroying weaponry; able to carry all your friends and family around in one open palm; everybody will take you seriously/nobody will ever laugh at you again
Cons: virtually impossible to hide, even powered down; major collateral damage an inevitability; maintenance costs astronomical; newfound respect from acquaintances may actually be fear of your wrath