What commute bike is better for fitness building

I've got a really short commute but with hills at both ends (I'm in Bristol) I ride a fixed track bike with a 76" gear or I can ride a Surly 1 X 1 SS with slicks and a 61" gear
Which is best for general fitness building, spinning or churning along with the bigger gear?
Which is best for general fitness building, spinning or churning along with the bigger gear?
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A fixed, in theory, will be a better workout than a SS, also up the hills will be harder in the bigger gear. Obvious choice, for me