We will be flying in on a Saturday for a Sunday cruise - staying at a hotel near the airport for the night. What is our best bet to get to the peir - taxi, towncar service or rental car?
There will be 2 adults and 2 older kids plus at least 3 large suitcases.
We will also need transport back to the airport hotel on the Monday afternoon (staying downtown on the Sunday so shouldn't need a rental car)

I think a taxi would be easiest, your hotel will call one for you. With a rental car you'll have to mess with parking it, finding your way, etc. You won't need a car on the day you'll be staying downtown.

So do you think a taxi would be more cost effective also then getting a town car service?

Given the amount of people + luggage, 1 taxi may not be enough. That might alone be worth the town car service. Unless you can get a van for a taxi

Ok thanks - do you have any town car service you can recommend?

Plenty of taxi services have minivans. Don't know how much a town car is, but airport to the pier should be in the neighborhood of ~$45.