Signal strength isn't a big deal. My house is in one of Verizon's weaker spots in the Phila. PA 'burbs, and the DINC2 is better with this than the BB Storm before it. In general, we find VZW's service to be amazingly good in places where any signal would be a surprise. In our boat, we've been from the Chesapeake Bay to Florida and on to the Bahamas, passing through some areas where it might seem even electricity is scarce, and there's a Verizon signal. There are some holes, to be sure, but in general, I wouldn't sweat signal strength.
There are two real issues to think about when thinking IP4 v. DINC2. One, go with IP4 and you're going to get the whole Apple experience, like it or not. With DINC2, you're probably going to get Verizon, which has its own ups and downs. The biggest down, in my experience is that, in the US, you're going to get a CDMA connection and that means "no data while talking". The DINC2 is GSM capable (works in Europe, for example), but here in the US you're stuck with CDMA.
Yesterday, while driving to a new location, and using the DINC2 as my car GPS, I got "data lost" gripes from the GPS because I was one the phone (hands free, of course!) at the same time. The GPS Navigator uses the Internet to source its maps. Talk on the phone and that goes away. (Yes, Google Maps has a lab feature allowing map tiles to be stored in advance - they cover a relatively small area, and contain only map data and no traffic, satellite, POI, etc. info)
Oh, yeah, one IP4 oddity: in the DINC2, I can stuff in a 32G microSD, use it, make a backup to another microSD (externally, of course), and put in another microSD if I want a fast change of tunes, video, etc. In the IP4, the memory is sealed in the phone. What you see is what you get, and you can't change it. For most people I expect this is no biggie, for some it could be a deal breaker. YMMV