The 150 is larger, which is a plus.

The 250 is smaller, which is nice for panel space. It's about the
same size as a Garmin 430.

But where the 250 excels is that it can accept datalink for weather
(from the king KDR-510 datalink receiver) and traffic (from the King
KT-73 mode S transponder). That whole package(the IHAS 2000) would
cost around $10,000 plus installation. However, I'm not a fan of the
King products. The datalink is ground based, which has it's
limitations of line of sight if you require weather all the time.

The 150 is a dead-end solution with no expansion other than the WX-500
stormscope, which is why it's so cheap ($3000) with such a large
display.

Both have terrain display, but as posted previously the 250 shows the
terrain relative to your GPS altitude. That's nice. For the extra
money, I'd go for the 250 despite the smaller screen.