Dear God, first of all, a Graco Turbobooster is HORRIBLE advice. Boosters are for big kids...older than 4, bigger than 40 pounds, and mature enough to sit up and mind the belt. Under no circumstances should a two year old EVER, FOR ANY REASON, be in a booster.
That being said, how much does your two year old weigh? If he's under 40 pounds, ideally, he's in a rear facing convertible. If he's forty pounds or close to it, a forward facing combination seat.
The Britax Marathon is not at all a good choice. It's a convertible only, and it only rear faces to 35 pounds. It also has shorter top slots than other forward facing seats.
If you're going to rear face, the Graco My Ride, the Sunshine Kids' Radians, and the Safety First Complete Air all rear face to 40 or more pounds.
If you're going to forward face, keep him in a harness that goes higher than 40 pounds. The Cosco Apex, the Graco Nautilus and the Britax Frontier all will keep your child harnessed for several more years AND as an added bonus convert to a very good belt positioning booster. The Graco Nautilus is probably your best bet...affordable and a very nice seat. (Again, it would be substantially safer to keep him rear facing.)
~CPST