The F-Type looks to be holder value better than the XK that went before. Depreciation is surely simply linked to supply and demand. If they're making too many, too quickly, this will drive the value downwards. Add in a car that is now 3 years to market and so no longer quite to exciting for some and this drives values down further.
Maserati has never made enough cars to see this really happen.....until now ! Look at the value of the Ghibli and QP6 and you'll see similar trends to Jaguar developing. For example, a 2014 (3 years old) diesel Ghibli is now £25-27k. That's a car with a list of £51k, even poverty spec ones have a couple of options so let's say £55k average. Thats a retained value of 45% (ish) which is not impressive. The QP6 fairs very similarly with a 2014 diesel at £35k and a list of £71k, plus options so £75k. Where is appears that the QP6 is plummeting in value, it's actually doing the same at the Ghibli. Interestingly - and I have been banging on about this for ages - the petrol versions of both cars suffer less depreciation. A similar (non S) petrol 2014 Ghibli will set you back about £35k right now, maybe a little less. Add the £54k list plus similar options to the diesel and that's £58k. Therefore the retained value is 61%.
Bottom line is, don't buy a diesel one unless you are doing ballistic mileage. There's just no point. The petrol is a better car, better to drive and depreciates less. The numbers don't lie.