I had quite a detailed conversation with my Rheumy, then followed it up with some research.
According to both, biologics are best injected into fat because it will tend to provide a more consistent release of the drug into your body. I didn’t research MTX as I always took it orally.
For me, therefore, my tummy had a better reserve of pure fat cells under the skin, so in theory that works better, and that’s what I do. Also I tried my thigh once and it stung quite a bit more. It would probably depend on your body composition though - in theory I should be injecting it into my butt cheeks for best results - but practically that would just be too funny !