The Bellerophon is T4 (.5) while the D'Kyr is T5, I think in principle there's nothing speaking for permanently keeping the Bellerophon.
Of course the D'Kyr costs CPoints and I won't encourage anyone to buy stuff for real money they might end up not liking.
So perhaps you might want to try the DSSV (not the prettiest ship I know) but I found mine to be quite nice.
For STFs, I used the Bellerophon at first but have switched to the D'Kyr and found it much more useful. The Ltcom engi slot is used for aceton field (a great skill against tac cubes that many people in cruisers seem to not have for some reason). The main advantage the Belle has is the ablative hull armour but I found that by the time you activate it the Borg have already destroyed you
I liked the Bellerophon myself due to it being an intrepid variant. i love the intrepid, and the console that comes with it is nice, but limited to photons. the d'kyr is a good ship, so i've heard, but i dont see many of them around.
I'm an Eng that mostly uses the Galaxy X class ship. But I take to my LRSV every now and then, a beautiful intrepid class. Using the Belle, I would say would be better then the D'Kyr, as the majority of the sci/tac/eng I see using a sci ship are eiter using the LRSV or the DSSV, I don't see the D'Kyr much, and actually think most players on STO have a missed opportunity with it because of the drone which is(quite possibly) useful for running the STFs
Edit: As a recommendation, I would get a set of combat engines, as they tend to be more efficient at lower power levels, increasing your speed and turn rate(I believe)