Big Block or Small Block? ... That is the Question
This is something that no one can answer *for* you, but they can certainly assist you in seeing the differences you should consider when making your powerplant selection.
Recommending the level of power to someone is like recommending the type of girl you should marry. Only you can or should make that decision, because you are the only one that has to live with it. ;-)
What are *you* comfortable with? TRY it first and let your first impression be the judge. If you're not used to big power in a small car, be careful... If you *are* used to it, then still be careful!
Choosing a big block or small block is like shooting guns. You might have a .44 magnum and not hit a thing, but be deadly accurate with a .38. Then which would be better... for you? This might be more the reason the small blocks are said to be better on the track than their slightly larger brethren. Less fear of the car itself, not the actual handling differences some might think. This is especially true in the very rigid and well handling Superformnace, which doesn't seem to differ much from small to big block any way.
However, if you are good with that big .44 magnum, you may never have to use it and which is why at car shows and on the street the Big block usually are respected as the most intimidating thing out there, and may very well keep you from HAVING to prove it, just by having it under your hood.
You can make nearly as much if not slightly MORE horsepower with a well built small block, however, usually you can make that same power more reliably and cheaper with a Big Block (429/460 series) because the inherent strength is already there in metallic size, not structual expense. On another note, the FE (427/428 series) Big Blocks will cost significantly more for the same or less results, due to all the extra upgrades in parts and oiling modifications reuqired to make this very vintake engine design live. Do not skip in this very important priority of concern with the FE series engine, ever.
But of course, if you are really not going to use all that extra power, then that power can be more of a nuisance than a benefit... kind of like having a dually long box four door crew cab pickup and only tow a trailer once a year. For some it would still be worth having it. However, for many, it wouldn't... again, the personal preference is the final factor in any decision such as this.
I have both a small block Superformance car _and_ a big block Superformance car. They almost weigh the same (125 pounds difference), feel the same (in ride and street handling) and handle darn nearly identical in the turns (not hardcore racing, but spirited street and country road cornering, etc). However, that's where the similarities end.
The big block's sound is deeper, however not different enough to tell people it's different other than trained fellow cobra owners. The G-forces the big block puts out is, well, darn right scary and certainly dangerous in the wrong hands... even the right hands on occasion. The small block is a little more forgiving with throttle "mistakes", and may have an edge in short course auto crossing events where every single pound counts when racing against Miata's and the like.
The Big block I have (750hp) gets 17mpg at best, and averages 12-15. The small block I have (450hp) gets 23mpg at best and averages 13-18. So the fuel economy between one and another is not all that significant alone to base your final decision on unless you plan on driving cross country an awful lot.
All engines will respond differently depending on their build and how radical their cam/porting is, so these numbers can vary greatly, but usually in proportion to each other. Neither is better or worse, but again, it's really abut what *you* want, what *you* expect and what *you* are comfortable with, and certainly, what *you* can handle. Now is not the time to use the machoness we guys all are born with... because saying, "ah, I can handle it" may be the last words you speak.
And remember when making your decision, that you may not be the only one driving your car... if your wife is not comfortable with driving it, that may take a lot of the fun out of a previously thought of "joint purchase" into what "you bought yourself"... and you know where that can lead! Remember, you need an excuse to buy more cool stuff for "our" Cobra in the future, not "more crap for YOUR car"! These words have been repeated by many a spouse in the past... heed these experiences.
The best advice anyone could ever give you is to take a ride in as many Superformance's as you can of all engine types and levels of performance including powerplant types such as a Mild small blocks, Radical small blocks, Mild Big blocks and a Radical Big blocks.
You shouldn't base your final decision on price, durability or fuel mileage of any of the choices, since they can be made very similar in the end result. Use them as factors, but not as the deciding one.
Unless you are hard core racing, I wouldn't use a few pounds either way as your decisive factor. First, lean towards what you "want", or have wanted all along. Then go out and prove to yourself if that's will be the right choice in the end. Sometimes just the underhood appearance is the factor all in itself. Sometimes it's the "drivability" of being able to hop in your car and take a 15,000 mile trek across the country like this couple did.
Just be sure the "negatives" of your choice one way or another do not outweigh the positives. After that, there is no question that you have just made the right decision.
Please note I did not suggest which engine you should seek one way or another, because *you* should be the one making those decisions. Besides, I myself couldn't decide as well... so I got one of each! (although, I do have my favorite, but you shouldn't be making your choice on what my personal preference is!)