For me, the difference between an H420 and a CTL-480 was enormous.
The H420 tablet itself is pretty solid. The pen, however, didn't suit me at all. The AAA battery added far too much weight and felt clunky.
The pen that came with the CTL-480 feels way more natural. However, the tablet has a coarse surface. If you decided to buy a CTL-480, make sure you get a tablet protector (I got mine from POSRUS through amazon). The protector will reduce the friction substantially allowing for more freedom of movement. Also, the pen nibs will wear down at a much slower rate.
In terms of software, I greatly prefer Wacom's. The H420 software doesn't allow you to input numbers for tablet area which makes it difficult to see the proportionality. The Wacom software also allows you to force proportions which can be beneficial if your display resolution is of the same ratio as your Osu! client.
If you plan on using the tablet for more than just Osu!, then I'd recommend something other than the H420 as well. The CTL-480 has a radial menu which is very customizable and is great for graphic design. There may be some better options on the market, but I haven't tried many other tablets.
People will always say that a new tablet, or just a tablet in general, won't make you better. This is true to some extent. However, if a tablet feels more natural for you, then it's worth the investment in my opinion. Personally, I find playing with a tablet to be much more fun. Sure, I could probably do just as well with a mouse after some practice, but it all comes down to what's more enjoyable.