The Synology DS918+ is the successor to the DS916+ and features a new CPU, more RAM, a pair of M.2 slots, and trays with locks. Strangely enough, the older model seems to perform better in real-life scenarios; however, the DS918+ is fast enough to still meet the increasing demands of a home user.
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips