Newbigging is a bit out of the way and I don't think has good public transport links. We've always used a community mini-bus if we wanted to do activities outwith centre. The accommodation is good though electricity can be expensive if you are staying in the Brownie House in winter. They are now offering Archery as an additional option which we found very good. They also have 3 cabins beside the campsite. Not been to Ladenford Den and not camped at Newbigging.
Feeling a bit disloyal for saying this as a Dundee guider, but the Ladenford campsite location is nicer in my opinion. It's a bit more interesting with a stream and woodland areas.
Newbigging is very well equipped, and the archery is good. The wet weather shelter is better at Newbigging, and the wigwams are a good option for smaller groups.
I suppose it depends on what sort of activities you want to do off site - there's a swimming pool in Forfar which is quite close to Ladenford, but I don't know about the transport links. At Newbigging you are closer to Perth than Dundee.
Both sites are good, having camped at both several times but for me Ladenford Den does have more appeal. I may be biased though as an Angus County Guider Don't think that public transport is an option at either site but we usually have a mini-bus or two. From Ladenford we have been to Monikie Country Park which could easily be a full day trip. Great high ropes course, sailing, kayaking, windsufing etc. Local coach companies in Forfar have very reasonable rates. Also Dundee again not that far for great pool & HMS Discovery. You would have a great time at either site though......