I would suggest you to travel by yourself if you have good driving skills . This place is very beautiful and I guess one needs his own time to enjoy the trip thus I did it by my own and it was hell of a journey . Have a look at my travel vlog to experience Spiti Valley and Kaza .
If you don't have any prior experience in traveling alone or any friends ego have taken trips inn their own to accompany you is advisable to go through agencies. if you are brave enough you can always plan you travel and there well be lots of travelers everywhere so no need to be scared
Answer depends on if someone in your group has been to Spiti earlier or not. If yes, you'll get a perspective on how to go about things. If not, suggest to stick with an agency who can get you a good driver. Hotel bookings can be done by yourself.