If I was you, I'd start cheap, experimenting on the way. I had a years of good performance of Browning BR-6140 on 20' mast over single-story house (about 30' height over the ground). My setup also included 40' of LMR-400 and 40W radio, Kenwood TK-880H. I live in a center of a mostly flat valley circled by the mountains, and this setup allowed me to talk reliably to another mobile TK-880H in the car, and less reliably (local dip, buildings, etc) to handheld. The max distance on a flat area was 8 miles or so. Of course, when you start to climb up on the valley rim, you'd have crystal clear communication with handheld as well. BR-6140 is a unity gain, not 6dB.
I now have Diamond X50-NA (at 27' over the ground), which is a ham antenna, and have worse SWR than BR-6140 on GMRS frequencies. It works about the same, though. Diamond X50 is a unity gain too.
The repeaters is always a good option, because you would not need to invest on base station, antenna, etc. However, repeater it is somebody else's radio, not controlled by you.