I think there about even performance wise. As for going fast and modding it it depends on what you want to do. You could put a nitrous kit on the lt1 for cheap or get every day performance and put a s/c on the cobra. I also think the cobras look mean, the sharp edges of the pre-98 comaros firebirds are awful.
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[b]Fikse FM5's, Upper and Lower CA's, Lowered, Upgraded rear sway bar, Steeda Caster/Camber plates
if you do a search they did a straight up comparison of a 96svt, a 96 ss, and a 96 ta. The cobra finished right between the marrow and the firechicken.
Either one is a good way to go honestly. You can get much more n/a horsepower out of an LT1 then you can out of the 4.6... and they will take a monster shot of juice for track use.. but they arent very good blower cars because of the compression.. so if you plan to stay n/a or only plan to use juice as a power adder, the LT1 is a better choice.. if you plan on getting a blower.. the cobra.
sn95's are some of the most badass lookin modern mustangs. i think if your looking for just flat out power, an LT1 could be better in terms of potential. however, they're not exactly the best looking when compared to the cobra. the cobra isn't exactly lacking in power though.