PWMTrav

If there are more than 2 of you, or any kids, and the price doesn't bother you, I would opt for a car/taxi. YMMV, but I have a younger child which means we have luggage, and I don't particularly enjoy the bus -> train -> train/bus transfer.

Based on that location, though, there's also a Hertz location ~3km to the east. You could get a car that morning, drive back to the hotel, pick up your group and jump on the highway. Return the car at BLQ. You should double-check this, but I believe where you're staying is a somewhat less restrictive ZTL (only weekend nights in the summer). A one way car rental might be cheaper, not sure. I'd personally just take a taxi.

If you opt for a train, just note that you're about the same distance from Santa Maria Novella (the "main" station) as you are to Campo di Marte. If you opt to leave from the latter, I wouldn't walk - the streets are busier in that direction. Basically, if you want to walk to the train station, leave from SMN and give yourself 30 minutes at least. Google Maps will give you walking directions to Campo di Marte, but I'd just again suggest not doing that.