If you ride long and/or hard or want to keep your bike for a long time, or if you simply appreciate well-made machinery, do not settle for less than 105. Low-end Shimano equipment does not take abuse or heavy use very well. (In contrast I have 30-year-old Campag. Record hubs with over 30K miles/50K km on them.)
Any differences in component weight or bearing friction will probably have little or no impact on your speed or efficiency.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069