Here's my take on the primary differences between BattleDress and Combat Armor:
The fact that Battledress is "powered" means that the fatigue is greatly lessened on the soldier from not having to lug around the many kilos of weapons and gear he is schlepping with him. If you are wondering what I mean by this, try lugging around 60lb pack (and that's light...), another 20lbs of gear (helmet, clothes, equipment bandoliers, weapon, gas mask). Now add in your flak vest (I think they are around 15-20lbs now). All of this gear is heavy, and moving around in it is very tiring after a while. I would have loved having a powered exo-skeleton in my days in the army!
The second thing I think that should stand it apart is the electronics that should be built-in. A TL15 BattleDress suit should have an extremely robust set of electronics - radar, 360 degree displays, a smart AI that you can program with pre-set functions, electronic sensors, etc. All of the neat and deadly things that you should be able to cram down and power into an armored suit.
So while some of these features could be added to combat armor, and I'm sure they would, in BattleDress they are all there, the soldier is trained to use them like he breathes, so he's gonna be more deadly (in theory) than someone who does not train to live in their armor.
As for the low-tech vs. high-tech debate... well, where to start! Like it was pointed out before, some things might be able to shrug off attacks, but can still be effected by lower-tech weapons. An early 37mm Anti-Tank gun has no chance of penetrating the Chobram armor on a M-1 tank. However... if you hit the tank from the side, those TL-8 treads are gonna be blown apart, the road wheels potentially damaged or destroyed, and you have a vehicle that is 3 TL's above you that is now disabled and out of action.
The same would go for space combat. Missiles would get better, smarter, faster, but a 40 kiloton nuclear detonation is a 40 kiloton nuclear detonation. So if you were able to HIT a TL 15 starship with a TL7 nuke, its still dead or damaged.
That's the fun of things... using your head to figure out how THIS or THAT might happen whilst you are playing.