This question is not limited to the installation of hot tubs and spas (whether on the backyard or in the bathroom) but also applies to whether or not you'd like to peel off your carpet and install hardwood floors, replace the tiles on your kitchen with granite countertops, installing a beautiful backsplash, state-of-the-art kitchen sinks, lighting fixtures, etc etc.
We learned that regardless of which one you plan to put in first, the key tip is to make sure that you are comfortable with the budget, have a buffer (because you'll never know what hidden costs will sprout in the middle of the project), and make sure you have a good workable schedule. Also do it one part at a time too. Never ever tear up the whole house all at once...that spells a major disaster. Accomplish the little milestones a little at a time, and don't get swayed with too much high-end things. Shop at warehouses, or direct purchase stores, see if you can find materials on wholesale, suffice it to say, never ever buy retail.
And of course, research...research...research. Never dive into the water if you don't know how to swim. Since I mentioned water...Never start a swimming pool project if you can't afford the maintenance. In other words, go for what you can maintain comfortably.
Good luck on that crib. I'll be back for updates on ours.