The standard lengths come with the cross brace already welded on. The full lengths come in pieces, and include gussets which must be welded to various points along the subrame and floor.
Yes, they will provide more stiffening than the standard lengths. The difference lays just not in the $30, but in the extra labor time it takes to install the full lenghth subframes. This means either more welding time if you're doing it, or paying someone for more labor time if they're doing it.
If your car is mainly used on the street, with maybe occasional trips to the track, the standard length will be fine.
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Moxie Racing