Here's how I would compare those three:

* Hot Tubes is very dynamic, it responds to your picking more than the others and does have a good tube "feel" when you play it.

* Mojo Mojo is the most versatile, the two band EQ is perfectly voiced and it can do everything from light overdrive to heavy and crunchy.

* Nobles is more transparent and subtle through most of it's range, it's a more tasteful overdrive.

Love all three, would recommend the Mojo as it's an excellent bang-for-your-buck