Lizgdal, “123 has an excellent GUI and extra features, and GTR1 is amazing. I prefer GTR1's backtest methodology, the coding quality is solid, and GTR1 runs magically fast. . . 123 has an excellent GUI and extra features, and GTR1 is amazing. I prefer GTR1's backtest methodology, the coding quality is solid, and GTR1 runs magically fast.”

I agree with your conclusions about GTR1’s backtest methodology being superior.
But I’ve been using both GTR1 and P123 for over 18 years and have equal confidence in both. To me the important differences are GTR1 backtests have the advantage of daily start data and detailed reports on variants giving a quick view on how returns fall off with depth.
P123’s basic Screener at $25/mo only has 5 Y of backtests and no access to ranking systems. GTR1 wins.

P123’s Backtest at $84/mo has 10 Y backtests (research suggests 10 Y or greater is best). Ranking Systems which are better than Screens. GTR1 is capable of building ranking systems but P123’s is user friendly. Many standard and easy to define custom universes (GTR1 and P123 create universes by excluding equities by features. My opinion GTR1 and P123 at this level each have their advantages (GTR1 is free and daily starts vs P123 ease of use) and I would want access to both.

P123 Portfolio at $125/mo has 15 Y of backtests. Adds modifying models, modifying Simulated Strategies, Short Strategies, Headging and AI Factor for predictive systems (with add on fees for CPU/GPU training and monthly predictions).

GTR1 still had the daily start and earlier historical data advantage but P123 has pulled ahead in every other aera. Robbie has outstanding capibilities but it is hard for one individual in his spare team to beat a team of full time developers working on the system for over 20 years gradually increasing their employee count to a current level of 8 employees.

P123’s Ultimate at $200 /mo has 20 Y of backtests, Optimizer functions, Rolling tests, Regression Functions and more AI Factor and Data access.

In my view neither GTR1 nor P123 are an easy way to outperform the market. The individuals that I have observed achieving this have typically had either a STEM or finance background and spent a significant amount of time achieving their expertise. Both the MI board and P123 have had a large number of short-term users that were unable to profit from screens and drifted away.