The phones for both are basically the same, they are SIP in both cases. Dect and WiFi use different radios, so you cannot have a Dect phone without Dect APs. They do not interfere with one another, so you can have 802.11a/b/g/n APs and Dect APs in the same environment.

If you are running voice over WiFi, your wireless design becomes a lot more important. You need to look at QoS, adequate saturation for roaming, no overlapping channels, and dropping the transmit power on the APs to match the WiFi phones. If you have issues with your current WiFi phones no roaming well, it may have everything to do with your wireless setup and nothing to do with the phones. Changing phones may not solve anything.