I don't think you can answer your question without knowing what you're going to turn. If, as a furniture maker, you're only going to make table legs on your lathe, the length of the base 3520 is just a tiny bit short, so that would make your choice easy. Since your 3520 has extensions, you're stuck with a dilemma.
For someone with a large shop, the biggest difference between the two is the ability to turn larger diameter pieces on the 4224. The 3520 will make bowls larger than your family will want, so what would you turn bigger than that? If you think there are some things, then maybe you keep the 4224. If you plan to make architectural pieces, like columns, newel posts, or flag poles, then the extra bed length of the extensions makes the 3520 with extensions the winner.
The clamp on the banjo is another fairly important difference between the two. The screw clamp on most 3520s tends to come lose at times, usually at an unfortunate time, and then the tool rest pivots to contact the wood. Not good. The clamp on the 4224 is a different style, that doesn't loosen, but it's a PITA if you change tool rests often, as the grippers down inside tend to slide together preventing you from easily inserting the post. Pick your annoyance.