You should check where you feel more comfortably working. For ethernet on PIC you can use Harmony or MLA. MLA is a discontinued product, but it's a nice set of libraries where it's fairly straightforward to implement ethernet on the PIC18. Harmony wants to be easier, but I personally find it more complicated and harder to use. Harmony doesn't support PIC18. So, IMO, find the TCP stack you are the happiest with and choose the device accordingly.