I owned an SX SJB LTD2 bass. I have test a couple of fender MIM and squire bass. Here's my 2 cents point of view. They do sound different due to the combination of materials and parts.

Here, it is difficult to make an apple to apple comparison as different retailer usually do not carry the brand that you can A-B test. Even if you managed to find a shop which carries both Squire and SX, it is unlikely they would have the different Squire range to compare (the sub-RM500 squire and RM1000 differs in sound and construction quality). You should test the bass with the bass amp that you intend to purchase. This is gonna get complicated as even Fender MIA will sound craps if the amp is crabby. Your expectation will surely be compromised somehow as the sound may not even come close unless you can afford to stretch your $$$ for the equipment. Therefore you should take a pragmatic approach:

1. Determine your budget allocation for the bass and the amp
2. Do a bit of leg work and go round seeking shops which have the brands for comparison
3. Start selecting the amp as the common denominator for test. Do not use their high end amplifier you can't afford to take home.
4. The amp choice depends if you want to play in room or gig environment (this will determine the power 20W-200w and the portability).
5. Check the selected bass for the workmanship which can affect your playing i.e rough frets, buzzing, high action, electronics , etc.
6. Bass string types (round wound, flat wound, coated and etc) affects the sound more than guitar strings.
7. You need to justify to your wallet if the 'better' sounding bass is worth the extra Ringgit you have to fork out (does twice the price sounded 2x as good?)

Most important is to let your ear decide so that you will not regret your purchase. Surf and net and check out this link on opinions: [URL=http://http://www.talkbass.com/forum/showthread.php?t=477183]
Hope this helps.