Very little difference. The larger ring and cog will give you slightly better efficiency and will take longer to wear out but I used to get perhaps 10,000 miles running 44-17 riding a commuter year round in all weather. The small combo is a shade lighter. On the bike of my logo, I run a double sided hub. With a 42 I can use a 24 tooth cog and get a respectively low gear for mountains. I can run a 12 tooth for the downhill which for a fix gear is a respectably high gear.