Well, I'd say it's the ICW-1000G that you should go for, not the older model. I note that the advert actual lists "fast L2/L3 roaming" as a benefit/feature - this ensures you can "walk and talk" with it without too much droping out (assuming you have good wifi coverage). Big advantage for users on the move.
I wouldn't necessarily recommend the M4 Aqua - we chose it because a) it's waterproof (we're a nursing home so all sorts of "water hazards" available) and b) it was on special offer from one of the UK networks as a PAYG deal for £80 (about $120?) and we're running them without any SIM card anyway. Any Android/Apple smartphone will run a SIP app including the one in your pocket 
- Zoiper, CSIPSimple, Grandstream Wave are all good (and free), Bria is under $10 and suits us because they have an iPad-specific version that makes better use of the screen than the others and has "office" features like the Transfer button when you're in a call.