very hard to get tickets for 6 nations unless you are a member of a club in those countries, belong to a recognised supporters group or are willing to pay for a hospitality package...
I am a member of the England Rugby supporters club and have to go into a draw for the right to buy tickets for the major matches... haven't been selected in 3 years.... If you can get tickets either would be good.....I suggest you try for both and see where you get lucky...Autumn internationals are a safer bet....and often a bit more fun