próxima, siguiente

Question

If I want to say, "I will send a photo in my next letter," would I say "Enviaré una foto in mi carta siguiente"? Or would I say, "carta próxima"?

Thanks for any help.

Answer 1

Both possibilities are correct:
"Enviaré una foto en mi siguiente carta", although "carta siguiente" is ok too.
"Enviaré una foto en mi próxima carta".
For me, it sound better the second one.

Answer 2

I'm pretty sure it should be siguiente. Someone who can explain why is sure to come along soon.