Lug-centric Bolt-on Spacers
The advantage of this attach is they can provide their own lug nuts to correctly install the wheel spacers on, which would save much trouble of adjusting the length of studs. However, they are directly bolted onto the wheel studs of the vehicle and don’t have the lip to convert the torque on the hub. The consequence is that all the stress goes on wheel studs. It is not safe, because obviously, the manufacturer did not design lug nuts to handle your vehicle—they manufacture the hub to handle that.
Lug-centric Slip-on Spacers
As for this option, I’ll not really recommend it to you personally. First of all, it is lug-centric, which means that it mounts directly to the lug nut without notches that connect to the hub. So all the stress is put on the lug nuts, which is not safe enough. Secondly, this type of wheel spacer is very hard to center the hub. The vibration can be easily noticed when your speed is more than 60 mph. Last, the same question as hub-centric slip-on spacers, you have to ensure the thread length.
Conclusion of Type Option
Given the reasonable distribution of stress, the hub should be the center of stress for normal drive. Make sure your wheel spacers are hub-centric instead of lug-centric. If you choose slip-on spacers, you must ensure your stud length to prevent the wheel from falling off. If you don’t want to worry about too much, just choose hub-centric bolt-on spacers. It’s definitely safe!