Hi, I've been trying to get into ARM microcontrollers for a while and I just found out how great STM32s are !
If I where you I'd check NXP's LPC series ARM controllers as well. IMHO these are better than ST's where it comes silicon bugs and to low-level software compatibility across their entire range so if you know how to use one, then you also know how to use the rest. This makes it easier to choose a microcontroller which fits a particular project.
Both ST and NXP/Freescale are pretty good to start with. The one thing that ST was lacking years a go was a nice free non stripped down IDE. Freescale had them beat in that regard (and not too mention that the API was much better than ST's). The IDE from Freescale was bug not free, but it still was better than what ST had to offer. However, ST bought Atollic and now this is not an issue (previously, you could get Atollic, but you had no debugger).
One thing that ST is doing now is developing there own eco system instead of just relying on MBED, which is sort facing an identity crisis at the moment. Because of that, I have to give the edge to ST for beginners. Yes, I like the NXP/Freescale libraries better, but ST has kind of made everything generic, even down to the hardware. Different lines of different chips have the same or similar pin outs. They even tell you in their manuals how to design your board with upgrading or downgrading across chip lines. In fact, all the nucleo boards of the same line are the same exact board, just with the chip replaced and maybe one or two resistors. The cubemx software is not bad (last I tried, the Freescale version that was integrated in the IDE did not work well, but this was 3 years ago). The other added bonus is the cheap JTAG that you can buy from ST. Also, when it comes to me having to outsource work, it seems that more people come forward when it is the ST platform. With Freescale/NXP, I really had to go digging to find someone.
In therms of silicon bugs, not sure I agree with that, but I have only used these micros for 5 years, so I am still kind of immature with them. However, as I mentioned, the API Freescale was much more accessible. Without using the any code generation programs, it took me about 10 minutes to figure out how to blink an LED. It took me an hour to do the same task (without cubemx) with ST.
In the end, the best thing to do would be to acquire a board from each and see how long it takes you to do a simple task on each (like blink and LED). Which ever one you like better and find easier is what you should use. At the end of the day, you are gonna be forced to use what your job tells you to use (unless you are the one that gets to pick). The much more important thing is to learn how to read the tech sheet and APIs. The whole use the most generic IDE thing does not really matter because both of those IDEs are just eclipse anyways and most hardcore firmware jobs use IDEs that cost money.