Hello Jozef,
Welcome to the ASF2 flight sim users support forums.
It appears to me that the Q400 has the most up-to-date panel and comes with a short flight tutorial. The HUD was just added so you will have to work that into the existing flight tutorial for now. Lots of bells and whistles, literally. There seems to be an unlimited number of alarms if you do not fly according to the manual. This discourages one from "jump in and fly".
The a320 has some new sounds, has the first cut at waking up the CDU for the sim - 20 new pages but not complete yet - you can follow along and check performance and such as you fly. This one also has a short flight tutorial but, has a lot of automation and not much flying - push buttons, turn knobs, wonder what is happening, just like the real world version.
The Learjet 45 has a very nice and clean panel and most basic flight instruments and gauges work but a lot of depth is missing for now. The latest update added the thrust detents and some numbers to look at for fuel flow and a few Crew Warnings are now present. You can view and zoom in on your flight plan and monitor progress and you can set the ref Vspeeds. The upper limit for sim flying is FL390 - FL410 even though the real world version goes to FL510 and most of those flights are in the mid to upper 40s. No hints from the dev when this might be addressed. CDU is dead, but has as a very nice FGC for AP,
The 737-500 is an older model 737 with round gauges but has a nice digital MCP for the AP. We only have the most basic information for this one, no flight tutorial, with a fixed weight (very light end) so we are left to guess the performance numbers. Still fun to fly. Max altitude is FL370 (real world and sim version) This one has the smallest engines available so the climbs are slow and time consuming but the cruise speeds are accurate. No fuel flow indications just yet, but could be in work soon. CDU is dead.
All four will fly a flight plan using the autopilot, fly an ILS approach and the controls are responsive. Be sure to visit the wiki pages and read about each one, except the B737 - no really useful information there.
These four planes could not be more different with nothing in common with any of the others so it comes down to a personal choice. I suggest you spend some time with each one and see which one likes you best. There is also a 747 but it is in a different class than these four.
Regards,
Ray