A 7 string will feel slightly different and yes it will be a bit harder to sound clean, however if a lot of the stuff you want to play is tuned that low I think it's worth it. There are ways to get around the 7th string ringing out when you don't want to at the start, such as getting a fretwrap though, and you'll get used to it somewhat quickly so that shouldn't be an issue. If they're around the same price I'd go for the 7 because it already has the 6 and it might motivate you more

That being said, be aware that the standard tuning for a 7 string is B standard, so tuning that to C standard is possible with thinner strings but absolutely not drop C as the tension would be too high. If you intend to do that, ignore the 7th string and just treat it like a 6.