Depends on what you want out of it and whether or not you want to mod.
When it comes down to looks I like Monotron...small, black, sleek.
When it comes to features, the 150 has more.
The filter is far better on the Monotron...no question about that.
The oscillators are a bit more pleasing to my ears on the 150.
The Monotron is a bit better for on the go purposes.
The 150 feels like it's more practical for live use due to the stylus.
The Monotron is far easier to mod.
The 150 seems like it has more possibilities when modding but won't be as easy.
Hope that helps!
MU Modular (Moog, Dotcom, STG, Analog Systems, and Moon modules), MS20 (x2), Microbrute, Electribes (EA-1, EMX), microKorg, AN1x, Nord Lead 2x, Blofeld, Alpha Juno 2, DX7, JX8P, Poly 61 (x2), Split 8, TR-505, Monotron Delay, Ace Tone FR-3, Stylophone