Hi, yes I probably should sell the Porsche but having owned her for 9 years it would be hard to sell; so undecided for now.
As to how the three cars compare, well here goes -
435d (2015) (in line 6) 3.0L twin turbo 309bhp (630Nm torque) Xdrive 8speed ZF auto.
987 (2005) Gen 1 (flat 6) 2.7L (NA) 240bhp (270 Nm torque) 5speed manual.
Z4 28i (2013) pre LCI 4 POT 2.0L twin scroll turbo 245bhp (350Nm torque) ZF 8Speed auto.
435d 0-60 4.7sec (official), probably 4.3sec on a good day. 4x4 Xdrive makes a difference off the line with launch control.
987 0-60 5.9 consistent (wet or dry), linear power delivery off the line allowing traction.
Z4 0-60 5.5sec +or- 0.2sec dependant on wheel spin.
The 435d is heavier in the corners with slight under-steer but epic on exit as Xdrive allows for early gas at the apex. Not so much a B road champ, more A to B missile. Not fair to compare with 987 or Z4, different type of car, does everything well with 50+ mpg on ECO Pro mode if desired. Overall the best of the three for overtaking, early power on pull-out massive torque (630Nm) delivery from low rpm giving significant mid range ability.
Th 987 has a very competent chassis / suspension set-up, ensuring great flat cornering ability with neutral mid corner manners due to mid engine layout; steering inspires confidence on turn-in & exit; when you run out of talent the porker might just save you. However to get true thrills you need to push this car hard, on the edge of grip she tends to drift on all fours (not back end happy), but at this point you are acting like a hooligan going too fast for road with little or no margin for error (fully committed for true thrills).
The Z4, The best for fun, not because its better than the 987, but because its not as good as the 987 !
Get the braking done a little bit earlier on entry; don't carry excessive speed, balance on throttle through the corner, be careful not too early on the gas at exit (she might just bite). Great fun. The Z4 28i (in my opinion) has just enough power & grip for fun.