I bought the original BMW OEM Winter Tire package (Dunlop SP Winter Sport 3D runflats) from another owner who was selling his 18" set (they wouldn't fit his new M5). So far, they sound and feel exactly like my 18" all seasons.
My retired E60 had 17" non-runflat Michelin X-Ice tires which I thought were very good.
17" or 18" shouldn't matter a lot but 17" will cut through the snow better (in theory at least) than 18"
2012 BMW 535i X-drive; Space Grey/Black Dakota, Fineline; Executive (Auto trunk, NAV, HiFi Prof, Lumbar, Rear Cam, SIRIUS, Voice), Technology (Blind Spot, Hi-Beam Assist, Lane Departure, Surround View), APPS, Rear heated seats, Tint.
Retired: 78 300D, 81 300SD, 87 190E, 88 Porsche 924, 89 260E, 93 300E, 97 Volvo 850, 98 E320, 03 BMW 325iT, 02 C230 Sport Coupe, 08 BMW 535i