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Were you planning a routed core or L2 core? If the former, VSS may decrease reliability, and/or provide worse performance in certain failure situations.
Assuming 4507 VSS supports quad, the 2nd sup per chassis does provide some additional redundancy that the 4500-X VSS would not. Other than that, the 4507 vs. the 4500-X just gives you the choice of different line cards. If you really don't need 2nd sup redundancy, and/or the various line card options of the 4500 chassis, the 4500-X might be a better choice.
The 3750X series is weak in the area of port buffering compared to the 4500s. Whether this might be a problem, depends on various factors.
The 3750X series is also weak in the number of 10g ports it can support, i.e. only two per 3750X.
The 3750X series is probably getting close to end-of-life, because of the 3650 and 3850s.
The 3750X stack, vs. dual 10g between VSS 4500s, offers more bandwidth; i.e. dual 16g, I believe.
The 3750X stack doesn't have some of the internal data flows "quirks" of VSS.
Either 4500 is more industrial strength than a 3750X, and it probably a better "core" device choice except for small networks and where device cost is a prime consideration.
