Shorter speaker cables are better than longer.
Shorter interconnects are also better than longer.
If you've got to go long with one or the other, there's no strong technical reason to go with long ICs (or vice versa). If you think one way is better than the other, use it--it can't hurt. If it's more convenient to go one way than the other, go for the convenient way--it probably can't hurt, either.
By the way, if you doubt what these cable manufacturers are telling you about lengths, should you believe everything else they tell you?
Shorter interconnects are also better than longer.
If you've got to go long with one or the other, there's no strong technical reason to go with long ICs (or vice versa). If you think one way is better than the other, use it--it can't hurt. If it's more convenient to go one way than the other, go for the convenient way--it probably can't hurt, either.
By the way, if you doubt what these cable manufacturers are telling you about lengths, should you believe everything else they tell you?