I own Marlin's in both calibers. And from what you describe as your wants I would go with the .44 Magnum chambered version. So I can highly recommend the Marlin model 1894 in .44 Mag. These can sometimes be found on the used market for about $350-$400. A Marlin model 336 chambered in 30-30 can be found used for as little as about $250-$300.
Overall .44 Mag ammunition will be slightly less expensive over .30-30, and at home defense ranges - it really does not matter whether you hit someone with a .30-30 or a .44 Magnum. They will go down. I will argue that a .44 Magnum is much better for HD as the concern of over penetration of the target is less than with a .30-30.
I like short barrel guns in general, however you could toss a coin to decide between 16 and 18". Practically, ballistically speaking, it does not make a difference. However of the two I would go with 16" just because you are looking at HD as your primary use.
NRA & NAHC Life Member