the T-Rex 450 is indeed a good step up from a co-ax, although a totally different beast to fly; there are cheaper models at that size as well.
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be aware though that the 450 size helis will need a room the size of a church hall to fly in indoors; they are also big enough to fly outdoors on a calm day
I disagree – far better going to a FP if you’ve only flown co-ax – the learning curve is a little less steep, and you don’t have to worry about all the setup stuff. You should be able to fly a 450 outside on a far less than calm day – I can fly my tiny 4#6S outside even when not completely calm.
I note you did mention some other options – out of those the SR120 would be my choice as it’s FP, but then personally I’d pick a Walkera with a 3-axis gyro over that (the gyro helps a lot with the stability) – eg the 4#6S I have, or the flybarless V120D01. That’s if you want something small enough that you can fly inside in a normal room and also fly outside. Then again, if you are happy with the next step being something to fly inside, an MSR or a Nine Eagles Solo Pro as they have a 45 degree flybar which makes them inherently stable, so a lot easier to learn with.