D3 Alone
D3 increases calcium absorption from gut. Without K2, that extra calcium may deposit in arteries instead of bones. Fine for short-term, normal doses.
D3 + K2 (MK-7)
K2 activates osteocalcin (Ca→bone) and MGP (prevents Ca in arteries). Long-term D3 without K2 theoretically increases vascular calcification risk, especially at doses >2000 IU/day.
Verdict
Always pair D3 with K2 for long-term use. The cost difference is minimal (~$2/month). Natto provides K2 naturally if you can stomach it.
Best choice: D3 + K2