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Adding to Dirk's Answer

Be aware, mounting on both sides of a board may not buy you as much real estate as you might be imagining.

When it comes to board density, your ability to route traces tends to be a critical factor as density goes up. More layers helps, but then you fill up space with vias.

Double sided tends to make it MUCH harder to route unless you use buried or blind vias and use many MORE layers. The cost tends to go up a notch or two and reliability goes down.

Also you need to be very aware of thermal issues, you do not want a device at 100C or even 50C on the back of a chip with lots of pins or a temperature sensitive analog circuit.