If you haven't used keys before, it'll slow you down for quite a bit until you apply some drive and determination to learning the keys. The possibilities for musical expression and rhythmic nuance are greater with a keyboard, but the learning curve is slow.
TBH, though, wayyyyyyyy back in the day when I bought my first 25-key MIDI controller, I just bought it to audition presets in VST synths and such.
For $300 or less, I'd personally go with a Novation Nocturn 49. The SL mkII 61 I'd normally recommend is twice your price range, but the Nocturn 49 has the same awesome keybed and has automap.