I've owned a 1973 911T targa and a 1972 914 2.0. I drove the 914 on the street for several years while building it into a PCA stock-class race car.
The 914 is a neat car. I enjoyed the open interior and loved the dash board and the VDO gauges. The interior is spacious yet, with the firewall just behind you, is also cozy. While I am not a convertible person it was fun to remove the roof on nice days, while the rest of the time the hard roof is nicer than a fabric convertible top. It tickled me that the view of the back end, especially with the lower bumper surround removed, reminded me of the similar view of the (also mid-engined) 917. If you want the feel of a sporty VW the 914 should satisfy you.
The 911 had sport seats and triple Webers; I'm guessing it had been autocrossed sometime in the past. I bought it not running, got it running again before soon selling it (in 2003, before values took off, sadly).
Yes, both cars have air-cooled, boxer engines behind you and, in my case, both were targas. Both have the sweet dashboard with the awesome VDO gauges. Both cars are from the same period in automotive history. Both of my cars had Weber carbs. Both tops leaked, no matter what. But boy, they are not very similar.
The 911 engine loves to rev and sounds awesome. The 914 engine, even rebuilt with 9.0 compression, dual Webers, electronic ignition and headers, didn't like to rev and always felt like it was out of its' element, and maybe even like redlining it was abusing it. So the 914 will feel like a hotrodded beetle but it won't ever feel like a 911. The 914 was built by VW to be a VW and it feels like a VW. The 911, exactly as people say, is a hand-built car constructed to high standards and it feels like it. The doors close with that sound, even a targa. The interior is miles nicer than a 914. The entire car seems to have had more money spent on engineering, and to have had more development time. The 914 feels like an old sports-car that you keep running while the 911 feels like an heirloom you preserve for future generations. I love the air-cooled cars (I had a Beetle in high school) and I love 914s but they are very different than 911s.
I love mid-engine cars and the 914 definitely has those mid-engined advantages. It was awesome to be able to out-trail-brake 911s into decreasing radius turns like turn 5 at Pueblo and turn 3 at Mid-America due to the superior weight distribution. Each car enjoys a unique exterior design. The 914 has motorsports heritage but, obviously, nothing like the 911 has. The 914 has a following but not like the 911. It has been good to see the 914 get more respect and to see its' values appreciating.
Their similarities are more obvious; I hope this gives you some perspective on their differences.