WEll, I can honestly say I've owned a few of each. 2 4.9's in '92 and '95 devilles, a TBI 4.5 in a '89 Eldo, and a PFI 4.5 in a '90 Eldo. The are all remarkably similar, especially the PFI 4.5 and 4.9. I'd have to say that my '90 4.5 PFI Eldo feels the fastest, though it's a smaller car than either of the 4.9 powered Devilles. In terms of reliability, the TBI 4.5 wins hands down. No injector or FPR troubles, no bearing thump, and just plain rock solid. While the others were reliable for me, there were a few fuel system repairs on the others that stop them from winning reliability over the TBI. Just search here for fuel injector and fuel pressure regulator replacement...
By the way, the 4.5 PFI and the 4.9 are near identical engines in every way. The only real difference was the 1-year connecting rods used in the 4.5 PFI engine and the crankshaft giving the longer stroke in the 4.9.The 4.5 PFI shared the block, pistons, heads, intake manifold, fuel system and ignition system (all new for 1990) with the 4.9. The only differences were the crank and connecting rods, which gave the 4.9 greater displacement. Externally the only major difference was the A.I.R system on the 4.5 that was deleted for the 4.9.