The specifications are built-in the chips that are used in the AP and the AP-310 has a different (newer) chip than the AP-320. The 320 can do 4 streams, when in SU-MIMO. I doubt that you will really see the difference in practice as it only attributes if there is actual data to send to enough multi-user mimo capable clients at the moment of the transmission. Don't expect 33% more performance from a 4x4:4 versus 4x4:3 in practice, it might be under laboratory conditions.
The same antenna element is used for receiving and for transmission. Remember that wifi is half-duplex, so an antenna will either transmit or receive at a single moment (and the radio can very fast switch between sending/receiving). If an antenna element is a dual-band, which means it is designed for both 2.4GHz and 5GHz simultaneous operations, one of each radio chains are connected to the same element. The 310 has 2 radios, with 4 antenna chains each, where each of the antennas is connected to a radio chain on both radios (with filtering).
From the datasheet of the 315:
5 GHz 4x4 MIMO (1,733 Mbps max rate) and 2.4 GHz 2x2 MIMO (400 Mbps max rate) radios,with a total of four integrated omni-directional downtilt dual-band antennas
From the datasheet for the 320:
5 GHz (1,733 Mbps max rate) and 2.4 GHz (800 Mbps max rate) radios, each with 4x4 MIMO support and a total of eight integrated omni-directional downtilt antennas.
I wouldn't pick my AP based on these differences or specifications though; there are more important things to look at, like the customer requirements, a possible redundant Ethernet on the 325 or the higher capacity of the 325 versus the 315. Please use your local Aruba SE to help you pick the right AP for each environment and match the customer requirements and budget.