Either will work but there are practical and electrical differences to consider.
Stranded wire with a diameter over 16 AWG is easier to work with since thicker solid wire is too stiff. But either type will work for a PIT tag antenna.
Solid and stranded wire have different electrical charactistics. Each strand is a separate pathway and a multi-strand wire will have lower electrical resistance versus a solid one. Wire composed of many fine strands has lower resistance than the same size wire made with fewer thicker strands.
When describing a wire size, a complete specification is three numbers, the overall size (AWG, MM2), the number of strands and the individual strand size. Some manufacturers use two or three different strand sizes in the same cable.