The difficulty with the 150 mm OTA is that it's hard to get an acceptable mount at an inexpensive price, which is often critical when starting out. Realistically, you're looking at a minimum of about $1400 for a 150 mm OTA & minimally acceptable equatorial mount (e.g., EQM-35). If you really want the simplicity of an alt-az mount at this size, your best bet is perhaps the IOptron Urban 150 bundle for $1900.
The rule of thumb when imaging is to get a mount with capacity about 2x your actual payload (OTA, finder, camera, etc.) The Orion Starseeker IV /150 mm bundle at $1000 is attractive at first glance, but you'll easily be above the capacity of that mount.
OTOH, the various alt-az mounts that come with these 127 mm scopes are adequate, easy to use, and hit a beginner-friendly price point. The real bargain is the Celestron around $450 (street), although the 127 Starseeker IV bundle at $700 is the same OTA with an upgraded version of the mount with dual encoders (allowing you to unlock, manually point, then re-engage tracking/goto).