I can see myself regretting what I'm about to say, in that I'll never hear the end of it afterwards, but MML are far, far better than FGW. No contest. Even 'one' has a better reputation than FGW!
Dirty MML trains? FGW have them too, just we have them a lot more than MML (in my experience). Especially after running a fully wedged service to Cardiff on the day of a big match (honestly, the HST I saw arriving and leaving at Newport, as well as being late, was very busy. Full and standing in First Class even. Extra train services FGW? You said you were running them, but I didn't see any sign of this!)
Slack times? FGW has this too. Pump the timetables full of it, but does it make much difference? No. The trains are still late, considerably so too.
Poor customer service? FGW in a nutshell that is.
You know things are screwed when you have to get the train before the one you want to get to ensure your journey is completed on time. Again, FGW in a nutshell, especially at Newport. Arrive from Hereford on time at Maindee North, and you get everything pass you. HSTs to Cardiff, FGW Local to BTM/Pompey, and of course the train you want to get on, the one which is a booked connection off your train, the one which has already arrived late at Newport, leaves even later and goes before you get routed into the station. If it doesn't, you can put money on going into platform 1, where the train stops right down the other end of the station, being a monsterous run up to the footbridge, across it and in time to see the doors locked in front of you... At weekends, this means at least another hour until the next train. Assuming it runs, and if it does, if it runs to time. Which is almost always never the case. So if you're planning on getting, say, the 1649 to Newport to make the 1739 off Newport to Paddington (a packed run as it is from all stations to Newport, gets insane after that), you aim at getting the 1609 instead to Newport. The fact the Cardiff-bound Holyhead trains (xx09 every two hours) run late fairly often (75% of the time in my experience) is neither here or there though in this case. If it's on time, you curse then, as it means a 40+ minute wait in Newport. This isn't particularly fun anymore too, especially if you go there on the days when freight through Newport is thin on the ground. But you do get to see if the 1649 arrives on time. Also, if you're lucky, weekdays mean you can get the 1709 if the 1609 leaves Hereford on time and arrives on-time (a tight 7-8 minute connection as it is off platform 1 usually anyway) at Newport, but you never plan to get that.