At your budget the Denon DL-301 MKII is a good value.
It might help if you gave a little more information about your system. If it's not that resolving, you might not notice a big difference between the carts you have, if it's highly resolving, it might point out some of the shortcomings of the DL-301 MKII.
You should notice more detail and a smoother presentation and probably improved bass with the Denon compared to the Shure. I have no experience with the AT440ml.
If you really want to get into MC, I'd suggest saving up and get a Hana SL, it's noticeably better than the Denon, but the Denon is a nice way to dip your feet in the water.
It might help if you gave a little more information about your system. If it's not that resolving, you might not notice a big difference between the carts you have, if it's highly resolving, it might point out some of the shortcomings of the DL-301 MKII.
You should notice more detail and a smoother presentation and probably improved bass with the Denon compared to the Shure. I have no experience with the AT440ml.
If you really want to get into MC, I'd suggest saving up and get a Hana SL, it's noticeably better than the Denon, but the Denon is a nice way to dip your feet in the water.