It's your choice, really. You might find opponents at lv 53 and lv 70, both have a pretty low chance though...
And you also get higher amount of credits for winning a battle at lv 70 than lv 53, the difference is about 850 credits per multiplayer battle if you're not a premium user. Halve it for private and singleplayre matches, or double it if you're a premium user indeed. If you're premium user and do a singleplayer or private match though, it's 850 again
And I'll tell you how much my lv 53 build costed me:
It is exactly
1.264.300 credits with every item, including the current kit set for it.(the rare torso "Dragon", two rare swords -"Devastator" and "Hell Force" - and one rare module-"Adv. Resistance Module V")
The Lv 70 build will probably cost more than
1.700.000 credits for you, it requires the lv 70 rare drone "Antares" to buy if you really want to be lv 70, or you'll be lv 68 or lower.