Both are signature based. Both decrease WAN-LAN performance depending on hardware capabilities. AiProtection needs data sharing agreement with Trend Micro before activating. Threat Protection is an App available in SRM, has to be downloaded, writes stats on USB media, more flexible (own rules, per device, etc.) and with more often updated signatures, but less user friendly. I have one Synology router in use, but not running Threat Prevention on it. Many users around with Asus routers run AiProtection and are happy with it. It's more user friendly, basically on/off for the entire network. Some users express privacy concerns with Trend Micro.
Which one is more safe? - No answer to this question, perhaps neither. Both won't see encrypted traffic and if the users are careless - won't do much to protect them.
If you ask about which AIO router vendor to go with - my personal opinion and experience is Synology. Perhaps more expensive, but with better software. YMMV as usual.