I am relatively new to owning a Mooney and bough my 67 M20F last year. That said, I have put a few upgrades into the aircraft. There has been some great advice from people here on this Mooneyspace. @Marauder Has helped me make some decisions as well, though I have decided to go a different route than him, his set up is great, and I plan for my set up to be solid as well.
Attached are a few pictures. The first is when I purchased my aircraft last year. L->R, Standard 6-pack, STEC 60-2 autopilot, Garmin 496, GNS 430W, JPI 830, and KT-76 transponder. My Vaccum system was decent but I wanted some safety measures. So I upgraded the ADS-b in/out adding the GTX 345 and upgrading the 496 to an Aera 660. This upgrade alone was about 7k installed. Traffic in and out (gtx 345), back up ADHRS (gtx 345), terrain warning (Aera 660) and back up/legal charts (Aera 660). I flew this for a few months and then the vacuum system started to go.
So... my gyros started to go and I new I figured an EFIS would be helpful and wished it to be integrated into my current stack. At this time the old aspen was cheap (6-8k+ install) and the new aspen was coming out (10k+install) and there was the E5 (6.5k+install). The dual Garmin G5's were available (5.5k+install). The Dynon system was a thought but seemed too far in the future, without knowing it would be approved for the Mooney. Most importantly I wanted SAFETY. So integration of the EFIS and Autopilot were the most important for me. So I looked more into the system lines, 1. Dual G5's and GFC 500 ~ ($5.5k + $8k + install), 2. Aspen Pro and STEC 3100 ~ ($10k + $12k + install), or 3. Dynon HDX Touch with autopilot (unsure of cost but suspect 18-25k + install).
Then I was thinking that we pilots are can screw up and safety is paramount. So integrative EFIS and Autopilot and ability to grow into those systems was the most important to me so.... The Garmin system is totally integrative, redundant and made to be integrated. The Aspen/STEC systems are independent systems and well integrated but expensive and nothing was STC'd or approved at that time, nor was the new Aspen shipping or STEC 3100 approved. The Dynon system IMO is the most integrative but the shipping/approval timeline unknown, and suspect the install would cost more.
So... for an immediate/6month fix I came to the conclusion that my preference order was 1st Garmin, 2nd Aspen/STEC, then 3rd Dynon as it was on the distant horizon. And if time was no factor Garmin and Dynon would be neck and neck but would need pricing for Dynon. I also had a lot of Garmin electronics installed at this time with Aera 660, GNS 430W, and GTX 345.
So...This is what I went with (Garmin) knowing that I could do two things. 1. Immediately have an solid certified EFIS with certified back up that integrates seamlessly with my current avionics. 2. Upgrade my Autopilot to the GFC 500 with a fully integrated A/P system, have Envelope protection, vertical controls, and TO/GA. So for 15k+ install, probably 25k total, I would have a State of the Art aircraft panel and control in a 50+ yearold airplane. And I could upgrade the navigator and audio panel and there would be NO integration problems.
If I wanted the same thing with the Aspen/STEC 3100, it would cost the same or more. probably $25-30k in my estimation. AND it would take atleast 6-12 months longer to be approved and installed. Currently @marauder has a great example and he may upgrade to this Aspen/STEC update update. Both are great systems.
So what does this all mean to you.
1. Know your mission (hard IMC vs day vmc).
2. Pick your budget.
3. Set your upgrade priorities (system integration, redundancy, etc)
4. Pick your timeline ( 1-2 months or 12 mos +).
5. Pick a system (STEC/Aspen vs Dynon vs Garmin).
If you want the "If I was you..."question answered... I would say Any of the 3 works, but the cost of a new GFC 500 $15k will be much cheaper than a new STEC 3100 install $20-$25k, and the Dynon is not yet determined. But if you can wait.... it might just be the best option for the best price.
Good luck and have fun!