After much contemplation about using my daily driver (Lumia 1520) and hearing all the concerns and issues people were having (after all its still a technical preview). I decided to purchase a 520 on go wireless of AT&T, and without subscribing to any plans, using wifi, have been able to update Windows 10 TP. I will say that I've only used a 920 and 1520 so I'm used to the processors having the robustness to deal with the taxing apps and required memory, but I was pleasantly surprised that the 520 wasn't half bad. I'm able to utilize all the new apps (new store, Xbox, music and video apps, voice recorder, which I love, outlook calendar, and the office suite apps).
I've you're stuck on the aesthetics of Windows phone 7 and 8, then you might be disappointed. I however love the universal merge that you see taking place on the Windows ecosystem. The store app on Windows 10 mobile looks just like the PC app. The office apps are amazing. I like where the Xbox app is heading. It's like Xbox smartglass and steroids. I like the music app. Some may say it resembles the player on iTunes, but it looks like a generic music player. I like it.
Ultimately if you're looking at updating to Windows 10 mobile TP..I would highly advise you to use a phone that you're not needing. I understand wanting a more spec heavy phone, but you'll be pleased with the 520, 521, or 635. All are capable of providing you with the Win 10 experience.