I wouldn't build a motor, or base any significant upgrades I had planned around a $250 intake manifold.

Decide where you want to end up after the upgrades, then the decision should get easier......and if you can put that m1 into use as part of the upgrade, great.

If you're planning on going towards the upper end of the 600-700 range, the 270's are the easier path...... provided the intake manifold associated with using those heads fits in with the rest of your plans.