Not really enough information, and I don't think you are exploring all your options.
About the acoustic: I was raised playing an arcosonic. Pretty nasty piano. Yours may be in better condition than mine was but it is an old and not very expensive piano. I later got a digital and it was much better in pretty much every way. Anyway, the condition of the piano is a consideration and without more information I'm going to have to assume that it is not very good.
About the digital: There's a lot more variety in the quality of digital pianos than people sometimes realize. They all kind of look the same, but the differences in the quality of the action are quite large. For example, the GHS action in a Yamaha YDP141 is very low end, while the only slightly more expensive YDP161 has a GH action that is much better, and almost certainly better than an old arcosonic in typical condition. Roland has their own style and various qualities, as does Kawai. Casio makes some actions that compete with GHS but none that really come close to an acoustic in my experience. So I guess the question is, what digital are we talking about? Since you don't mention a model and you call it a keyboard, rather than a digital piano, I assume it's low end. Are you considering buying a new one or just adding a software piano to it?
About the software piano: PianoTeq is one option. Not the best in my experience. Fun if you love tweaking the sound or talking about the innovative technology, but it doesn't sound as nice or as much like a piano as the best software piano options do. PT 4 seems to be an improvement over previous versions but it retains many of the flaws in the sound and isn't very pleasing to the ears. I've played many software pianos and the best I've played are Galaxy Pianos (including Vintage D) and Ivory II. Also good considerations: Garritan and Imperfect samples. If you consider other options just make sure they feature partial pedaling and repedaling or you may be disappointed in their playability.
Also to consider is what you use for speakers. Onboard DP speakers are often very low quality and not positioned very well. You can do much better. The speakers are the weak link for many (most) digitals. Compared to the big old soundboard in an acoustic, cheap speakers are very unimpressive. Try some high quality headphones if you want an outstanding experience.
Anyway, I find that a good digital with a good software pianos and nice set of headphones is very hard to beat. No cheap acoustic can match the experience, and it's convenient too (no bothering other household members while you practice, for example). It does depend on the circumstances, though. I can imagine cases where even a low quality acoustic would be preferred.
Since you already seem to own a Yamaha, a software piano seems like a good incremental improvement (at low cost). If you become dissatisfied with your action you can get a better controller and sell your current one on craigslist. If you get dissatisfied with the arcosonic (which I would) you will have to find someone willing to haul it off, which seems to be quite difficult in the current environment.