A comparison was gives elsewere in this forum.
The RT-AC87 and RT-AC3200 are different routers, each with specific features.
Different chipsets, different radios, 4 versus 6 antenna's, for quite sure different CPU's.
Main differences:
- RT-AC87: MU-MIMO (multi-user multiple input multiple output).
- RT-AC3200: tri-band MIMO (one 2.4 GHz radio, two 5 GHz radio's).
It seems the RT-AC87 uses the latest techniques and the RT-AC3200 combines "known" 802.11ac techniques in a creative way.
Other then bridging two equal routers, the application of either router depends on availability of matching network adapters.
Many network adapters (read: laptops and mobile devices) still have difficulties to fully support 802.11n, and even more to support 802.11ac in a decent way.
To my opinion you shall have real needs and know how to make use of the enhanced features of above routers before you buy them.