I want to ditto SoNo S totally. Her advice is great. If I were you I would consider the Graco Nautilus for your child to keep him in a five point harness to 65lbs. This seat will harness to 65lbs, then turns into a high back booster. IT looks just like the Graco Turbobooster, but with the harness. its a GREAT seat for $150, and so versatile.
As for the boosters, the Monarch has very deep side impact wings. Good for side impact protection, but some kids relaly dislike them. The Parkway is a GREAT seat, excellent padding, very comfy, great headrest. The Recaro seats are excellent as well, but may not fit older kids, so try it out first. The Compass seat is a great wide booster, will fit kids easily up to no longer needing a booster.
But we own a Britax Regent for our almost 6yr old, and LOVE it. We have a Turbobooster as his backup seat in our truck, but I honestly hate the thing. Its hard and not very comfy. The Regent will harness him to 80lbs, and hten my youngest can take it over until it expires. This would be the case in your situation as well, you could purchase the seat for your oldest, and hwen your youngest outgrows his current seat in a few yeyars, you could pass it down, nad then safely move your oldest to a booster.
BUT out of the choices you listed, I would go with the COmpass or the Parkway.