I honestly don't know much about this stuff at all. I've done some research, but it's all pretty confusing to me; VRAM size vs Clock Speed, Bus Width size, etc. 
I've been thinking of upgrading from my Sapphire Vapor-X HD 7970 GHz Edition to something newer, and both the MSI R9 390 and the MSI GTX 970 have caught my attention. The 390 has 8GB of VRAM compared to the 970's 4GB, but the 970 has about a 100MHZ higher clock speed. The 8GB of VRAM in the 390 is also 512-Bit vs the 256-Bit in the 970.
Of the two, which is the better choice? I'm trying to think of something that will hold up better, for longer, in the future. Like I said, I don't know much about this stuff, but to me, an extra 4GB of VRAM seems like it would be better than an extra 100MHz of clock speed.
EDIT: Here's some other helpful info.
CPU: i5-4670K @ 3.4GHz
Resolution: 1080p
PSU: 600W

I've been thinking of upgrading from my Sapphire Vapor-X HD 7970 GHz Edition to something newer, and both the MSI R9 390 and the MSI GTX 970 have caught my attention. The 390 has 8GB of VRAM compared to the 970's 4GB, but the 970 has about a 100MHZ higher clock speed. The 8GB of VRAM in the 390 is also 512-Bit vs the 256-Bit in the 970.
Of the two, which is the better choice? I'm trying to think of something that will hold up better, for longer, in the future. Like I said, I don't know much about this stuff, but to me, an extra 4GB of VRAM seems like it would be better than an extra 100MHz of clock speed.
EDIT: Here's some other helpful info.
CPU: i5-4670K @ 3.4GHz
Resolution: 1080p
PSU: 600W