Desktop is so much more functional than QBO is, see this non partisan comparison of QBO vs QB desktop
Be sure to look at the cost comparison in the above article, then add the monthly cost of any app you add (as suggested), an app to do what the software should do to start with.
Inventory in QBO is more basic than desktop, QBO is designed to buy and sell a widget nothing more. QBO uses FIFO, but there is no FIFO reporting to be had, reporting on an inventory item in stock by purchase date is what I mean, and adjustments can not address a date purchase which is required in FIFO inventory.
Multiple locations and Serial number tracking is only available in desktop enterprise with advanced inventory, and to be honest serial and lot number are poorly implemented. In enterprise you can have more or less serial numbers than you have qty on hand. You can enter a new serial number during a sale whenever you wish, a new number that was never recorded in serial number tracking. And some other things too.