My 2 bits...
a) What is your target board you will use for this research? Guessing that it is a commercial venture? If yes, review the displays based on resolution, other factors including back light intensity, viewing angles, touch panel (resistive vs. capacitive), interface - parallel vs. DSI. Budget for the end hardware?
b) Have not used STemWin so cannot comment other than a quick read which we thought was a restrictive license but free to get started. May be wrong here. TouchGFX used to be a 3,000 Euro toolchain but the company (Draupner) was purchased by ST and now is a free toolchain. TouchGFX is amazingly powerful. For our company, we were seeking some rapid tools for quick font use. Can do this in seconds and view the results. While TouchGFX is not perfect - no tool ever is - this solution is now tamed with the assorted documentation posted in this forum.
You can quickly perform your trials on the STM32F429i-DISC1 tool with SW4STM32 + TouchGFX 4.10 + CubeMX 5.0. The same now with STM32F746G-DISCOVERY kit.
Post back in the forum if you get stuck but the hard part for the above targets is resolved by assorted users over the past month or two.
If you do wish to take the end product live - be sure to review what is actually available in the market. For example, the TFT display used on the STM32F429 kit is based on a flex PCB with a 0.7mm pad pitch - not common at all in the industry. Respectively, this TFT display is hard soldered onto the PCB. As a company, we wanted to use a FPC socket in case the display is required to be replaced. The 48 pin socket @ 0.7mm is as rare as Elvis sightings. Instead we have tooled up with a 50 pin; 0.5mm pad pitch flex pcb and socket which are available from your local suppliers with ease.
Being a treadmill - I think I would personally consider a 800 x 480 with DSI interface and a CTP (capacitive touch panel). That would be a sweet look but bear in mind that the DSI display will be about $20 USD in Asia in 100-1k volume (perhaps a bit less) and that you will need to use a DSI supporting CPU from ST. Review the STM32F469I-DISCO as an example. Looks amazing BUT the display and supported GUI generated code is for an EOL display controller (OTM8009A). This target was our first choice but 9/10 suppliers in Asia shut us down for this request. They want us to consider a different display with a different controller. However, from some reading, understand that DSI control registers are common to each other but still is an uphill battle to support a new display with any of these tool chains. We have 3-4 weeks of solid and daily research invested in stabilizing the above tools so did not want take another battle.
Summary: STM32F429i-DISC1 and/or STM32F746G-DISCOVERY are good candidates with IAR / SW4STM32 use with TouchGFX + CubeMX. Each is confirmed to be working well now.
Other permutations will be life suckers...
