For original stuff, I'd personally go 7-string (my only guitar right now is a 7).
Some things to consider:
1. There will be an adjustment period which, depending on how determined you are, will take some time.
2. Depending on the model, some 7s are made with a different scale length than 6s, although if it were the case the manufacturer will state it clearly. This affects feel and sound, but may not be such a big issue for you.
3. Pay a little extra attention to individual string outputs, and adjust as necessary. The 7th string is thicker, has more metal, and is therefore naturally more magnetic than the thinner strings.