"The Best",...
is a controversial term.
I assume that you're talking about a NEW Porta 02mkII, for $149 retail,... in which case, I'd say the BEST idea is to SKIP the NEW Porta 02mkII, AND place the $150 down on either a good-condition-used 244, 246 or 424mkII. (Tascam). The 244, 246 & 424mkII are so much more of high end machines than the Porta 02mkII will ever be, often for a comparable price.
The Tascam cassette Portastudios are almost, as a rule, always better than any Fostex cassette-porta ever made. That's MO, of course, but the only Fostex cassette-porta I'd ever consider getting would be a 260, or maybe a 250, and even that's far cry from something like a Tascam 246. The X-12 is not on my radar.
IF it's simply a choice between the X-12 and the Porta 02mkII, I'd say take the 02, but I really think the BEST thing is a more robust, feature-rich vintage Portastudio. The 244 and 246 are prime examples of some of the BEST Portastudios ever made, with the 424mkII and even the 464 & 644 coming in right behind them.
I typically don't mention the Tascam 464 & 644, but they're as high end 4-track Portastudios as the 244/246/424mkII, that I thought I'd mention them, as a parting shot.
Shop Ebay/Tascam if you're not familiar with the models mentioned. Used analog gear is often your BEST value for the money. The more high end Tascam Portastudios will definitely prove to be more useful, in the long run, but the you'd probably find you'd outgrow the capabilities of the Porta 02mkII, very quickly.
Those are the BEST ideas I could give you.
BEST of LUCK
PS: In case you think I'm just blowing air up your skirt,... just 2 weeks ago, I scored a beautiful Tascam 244, for $72.50 + shipping, and it's in fine, low use, near-mint condition, that looks and sounds great. It's the BEST $72.50 I've spent in a LONG time,... but is my 5th,... yes,... FIFTH Tascam 244. It just doesn't get any better than that! Simply BLOWS the likes of the Porta 02 AWAY, & forget the Fostex at that point. No comparison. $72.50, BABY! See:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3725259869
