Indeed its personal what bike you like, but in general; a 450 suits all rallies. It also depends on your riding skills... when you're a beginner; start as light as possible. When a rally gets technical: get as light as possible. If a rally has long pistes and no (or little) really technical sections you can consider a 690 in my opinion.
And another tip; I see a lot of riders here with full rally fairings... this isn't necessary at all. Bigger tank, roadbook, ICO, GPS and you're done.