New York Airport Service from LGA to Penn Station...what's the transfer?

Am flying into LaGuardia and will be staying at at hotel near Penn Station. Looks like NY Airport Service is my most affordable option. Saw on their website that there is a transfer at Grand Central -- first, is it a move to a different NYAS bus or shuttle? Does it require finding the boarding point at Grand Central to continue the trip? Would it just be easier/quicker to hop on the subway at Grand Central over to Penn Station?