Deflection is primarily caused by bending loads, and is a function of the sectional moment of inertia of the beam resisting the load. The inertia is a function of the width of the beam times the height cubed, so the orientation of the extrusion is far more important than the series -- in one orientation, 3030 is only twice as stiff as 1530, but in the other orientation it's ~8x stiffer. That being said, deflection is also a function of how long of an unsupported span you have, so just some additional supports can help you save cost on extrusion.
There's a lot more that goes into selection of your framework (how tall your gantry is, cutting forces you expect to see, etc.) But for a practical answer, I can tell you that our kits are based on 1530 for the X, and 3060 for the gantry, just as you have proposed, and it works very well for the proportions we have settled on -- you can check out our plans here:
Good luck with your machine!