Maybe this is a silly question, but what is the min weight for this seat?
He's currently in a Britax Marathon (I think) that we want to pass down....Are there advantages to keeping my 35 pound 5-yr-old in the Marathon? Is he even big enough for the Pioneer?
2 years and 25lbs. http://www.britaxusa.com/car-seats/pioneer-70
Nope, no advantage. Any harnessed seat that is approved by the nhsta (so any sold in the US) is perfectly safe provided he's under the weight limit, his shoulders are blow the top harness slot, and it is installed/used properly. The Pioneer will last him a good long time.
With your Marathon, check the expiration when passing it down. I don't know how long you've had it, but Marathons expire after 6 years, so make sure you know when it will expire so you can replace it and keep the younger child safe too!