Confession first, I'm an iSheep so am biased.
I use an iPad with Runway HD. I have an external GPS, and use it, although always felt that the iPad GPS should probably be enough.
However, over Norfolk one day close to the Class D boundary the GPS dropped out. Started to blame the satellite as surely both systems couldn't fail ? Then found that yes, they can for different reasons. Battery discharged on the GPS and the iPad GPS dropping out.
As an aside, having the iPad and GPS rather than a panel mounted GPS does build in redundancy. If electrical power is lost you still have something electrical that works. I have experienced electrical system failure and a DI breaking on the same flight.
Back to OP, personally I'd go for a Sim iPad but not for other reasons completely unrelated to GPS or airborne purposes