While they are both reasonably good entry level boards, I would recommend the XLP 16-Bit Development Kit with the PICkit 3.
It offers a separate programmer/debugger for future projects, which do not utilize the XLP 16-Bit Development Board.
The device is socketed, which offers the added flexibility utilizing other PIC24 and dsPIC devices with 20-pin and 28-pin packages.
The development board in the MPLAB Starter Kit for PIC24F does offer more "bells and whistles," however I believe you'll find such permanent attached peripherals can actually conflict with designs which do not utilize them.
And these additional peripherals seldom match the exact components selected in future designs that you maybe tasked with developing.
The development board in the XLP 16-Bit Development Kit also offers a PICtail expansion connector, which I find quite convenient when developing many projects.
And Microchip offers a wide array of daughter boards which are compatible with the PICtail bus.
I would consider the board contained in the XLP 16-Bit Development Kit geared more towards development, compared to the board contained in the MPLAB Starter Kit for PIC24F as more educational.
In the area of true development, less features/peripherals are often preferable when considering a development board.
BigDog