The battery and screen size of the Z30 is incredible! I've had mine for just over a week now and absolutely love it!
Having said that, I not a big guy (5'8") but I find that the Z30 is a little too uncomfortable to use one handed. I'm constantly in fear of dropping it when I'm using it one handed. Never had that with my Z10. Definitely miss that from my Z10. Used it for almost 14 months before finally biting and getting Rogers to upgrade me to the Z30 for free
I don't think you can go wrong, or be disappointed, with either phone. Just be prepared to either charge frequently or get a battery pack for the Z10 as battery life is less than great. Even on my best day, I'd take my Z10 off the charger at 7:00AM and I'd have to charge it on my drive home at 4pm

but working in an office all day and spending 45mins in the car each way, I was always near a charger and never had an issue with plugging it in and topping up when I could. With the Z30 I've easily gone 24+ hours on a charge with my normal usage. Heck, I went almost 40 hours between charges this weekend. Took it off at 9:00AM on Saturday and only plugged it in at 11:00PM on Sunday because I was at 10% and figured I may as well charge it up overnight so I'd have a fresh battery for Monday lol
Now, that was with weekend usage as opposed to normal business usage, but still even on weekend usage my Z10 never last more than 12 hours
Like I said, I don't think you'll be disappointed with either phone providing you're prepared to deal with the shortcomings of the Z10 battery.