You're in conduit, use individual wires - they are cheaper, they pull easier, and the conduit fill on cables is terrible.
Typically we shoot for less than 3% voltage drop at rated current. Less drop is OK.
I'm fairly sure you need 6Ga wire minimum for a 60 Amp feed - given a short 30 foot run, this is also probably perfectly adequate. I'm getting 1.8% drop for 60 amperes at 240V on 30 feet (one way) of 6 Ga. Edit - Copper!
The limiting temperature rating if using THW, THHN, etc wire is generally the temperature rating of the connectors on the service equipment (breaker or panel) which is usually 75 C - even if using 90C wire, you have to follow the 75C section of the table due to the connections.
Copper .vs. Aluminum.
Copper (Cu) costs more, Aluminum (Al) less.
Copper has better conductivity - effect being, smaller wire to carry the same current. In this case, 6 gauge copper .vs. 4 gauge aluminum.
Copper oxides are conductive. Aluminum oxide is an insulator, forms quickly, and sticks very tightly to the wire. Aluminum connections need special procedures and materials to make a solid connection that won't get loose over time and overheat. These include things like brushing the wires with a stainless steel brush and an aluminum wire connection compound that coats the wire to prevent contact with air. The connections themselves must be rated specifically for use with aluminum, but most large ones will be (generally marked Cu-Al meaning they work with both, where unmarked connectors are assumed to be copper wire only.)
Aluminum connections are further complicated by cold flow, but that gets long and complicated to get into, and is supposed to be addressed by using the proper connector types. Essentially the connection gets hot, the wire swells, the swollen wire deforms, the wire shrinks, the connection gets looser, so next time it gets hotter, repeat until fire.
Best Answer
One thing to remember regarding voltage drop, it is heavily dependent on the actual current draw on the conductor. When you say that you are running 100-Amp capacity to the shop that means you are running wire that can handle that much current. That is not the figure to use to run a voltage drop calculation.
First make a list of all the items in the shop that you would like to run; include all items that are slated to be added in the foreseeable future, be realistic, include lighting and peripherals. Now narrow that list to those items that likely will run at the same time. Now determine the amp draw for each of these items. Add those loads together. You can add a buffer factor if you'd like. This is the number to use in a calculation for voltage drop. Happy calculating.