Take the cost of the hull+modules, subtract the cost of a "bare" hull, and you have the cost of the modules. Multiply the cost of the modules by the afore-mentioned 10-15%, and see if that's drastically more than the ship transfer fee. Make your choice, and wait for the ship to show up. If we're using a 100MCr as fitted ship where the hull costs 25MCR, we're left with 75MCr of modules. 10% of that is 7.5MCr, 15% is 10.75MCr. Say it would cost maybe 1MCr to transfer this ship and it would take 30 minutes to get there. Say you could earn 2MCr in that half hour doing an activity of your choice. 7.5-(1+2)=4.5MCr you'd save by parting out the ship ahead of selling the hull.
As an example anyway. YMMV, spend 2 minutes crunching numbers on a napkin and do what makes sense for you.