Technically, Swapmagic would involve opening and exchanging the top of your PS2 case if it's a PHAT model. The slims involve placing a few metal bits over parts in the PS2 drive, but not actually opening the case.
Swapmagics are rather cheap, as well ($30-40), so they're the most recommended non-modchip method around.
Personally I'm a big fan of 7th Style, IIDX RED, and DistorteD for their overall songlist quality. Pick one or 2 mixes and wait for the 2-disc EMPRESS+PREMIUM BEST release, which has over 180 songs listed for it already.
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