interconnect vs speaker cable

I would upgrade whichever I perceived to be the weekest link first. Aside from that, meaning 'all things being equal', I'd go for the interconnects first for several reasons:

1. Because of the low voltage being passed thru the ic's, they are performing a greater role when dealing with sensitivity. One little design flaw, impedance matching, veiling, constriction, glare, etc., here can go a long way. Assuming the replacement ic is 'better', I believe the ability to hear the sonic differences is potentially greater unless the speaker cable is inferior enough where they prevent you from hearing any improvements.

2. I have a feeling that because of the potential bottlenecking in the ic area due to low voltage sensitivity, etc, it is entirely possible and likely more probably to replace a speaker cable with a clearly superior cable, yet not hear any sonic improvements worth noting because of the restrictions in the inferior interconnect. And I suppose this argument can be used the other way around.

Of course I could be wrong on both counts above.

-IMO

Start with the interconnect. The signal path starts there.
Buy the most expensive cable you can afford, after a lengthy trial period of sampleing different brands and levels of those brands. I love Cardas, but not all their cables deliver the same benefits. Other considerations, assuming your tastes are as mine and that is I like a very analytical and detailed sound. I am not a "warm/mushy" listener. That said, try Harmonic Technology, Analysis Plus, come to mind as alternatives for a neutral non-colored sound.

The sad thing about this hobby is that "the most expensive (fill in the blank for any component here) that you can afford" is not always the best sounding product available to you. More often then not I've found that "more expensive is better" is rarely true, and the law of diminishing returns is a carnal law. In my own system I've cut the MSRP value almost in half on most pieces whilst dramatically IMPROVING the sound through experimentation with well reviewed cheaper components! Lots of hype in this hobby, lots of ego, only thing to go by ultimately is your own ears, which means you must try different things on your own at home.

That said, biggest change I've heard is with interconnects. But in talking any cables the sound changes very minimally, relatively speaking say a few percentage points worth, and how people write multiple pages or even a full page review of a cable boggles my mind.