Hey John, welcome! I put a link to what I recommend. In general, I would not recommend mixing wires of different gauges.
The wire I linked to below is what I purchased from Amazon after doing my research (NOTE: this is NOT an affiliate link and I don't make any money if you buy the wire). I decided to go with this because it's the gauge that Worx recommends (18 AWG) the same gauge as the wire in the box and made to be buried underground! It's also less expensive than the two you linked to and can be purchased in different lengths. It's solid core and is what I used on my yard in conjunction with the wire that came with the Landroid.