Seafile I think it's great.
Every great software, however, can not miss the following things:
1-More Feedback. People want to know and understand what the software is doing, at what point is reached, where it stops.
I tried just now BTSync and from this point of view does his duty. You know at what point is in each advance.
The speed of Seafile is more than satisfactory at the time of the transfer. BTSync does not offer the same performance.
3-The software even if it runs on the client side must be less resource-hungry and keep the same performance. With version 1.6.1 there was a rise in quality, but still needs to improve.
I hope to be helpful.