Is this better than the new Astrobot???

[Deathtank]

Or should I buy this now that's on sale?

[Chocos Ramabotti]

Yes it is.
Sackboy has some friction that Astro Bot completely eliminates in favour to blend in Playstations current fast food portfolio.

This is no joke, Astro Bot is the epitome of Style over Substance, while Sackboy has some really interesting plattforming, with plenty of ideas, still very good artstyle but most importantly, a proper difficulty curve, that doesn't start and stays at rock bottom.

Furthermore dying in Sackboy can have consequences with limited lives (which they allow you to turn off - not recommended) and several challenges that actually take some effort to fulfill. You need to amass points by collecting them efficiently with multiplicators, ignoring nothing.
There is a zero death challenge and plenty of challenging bonus levels, also the co-op levels put a high emphasis on cooperation they become pretty nifty later on.

Astro Bot might have more fanservice and is more beautiful over all, though Sackboy is still beautiful in its own and a way more competent plattformer.

Astro Bot has precisely 8 minutes of challenging plattforming (12 ultra short "challenge-levels" that are less than 30 seconds and a final one that takes approximately 2 minutes)
Sackboy has even in the first world some mildly challenging levels if you wanna collect all, like an autoscroller train level. None of that features Astro Bot.

Do yourself a favour, prefer Sackboy and wait till Astro Bot is more cheap.