I have run both stock units at the same time and kept the Fostex, but it's really what features are most important to you. Here are the three reasons I kept the Fostex:
1) Much easier to power externally. I think the Fostex will accept 6-12v, the 661 only 5v. I have powered the Fostex with a DIY 12v battery pack for more than 30+ hours on a single set of 8 D batts and still had 60% capacity left. If you record festivals, this could be a big deal. On the 661, you can use a lot of AAs, or get an expensive Tekkeon unit. I attempted to power the 661 externally only once. Used a set of 8 D batts (wired for 6v output) run down to 4.98v to avoid possibly frying the unit. I got about 90 minutes of record time before it shut down (4.75v). I was very fortunate to recover the file; most times, the file would be irretrievably lost if the deck shut down while powered externally. The Fostex saves the file each minute, so the most music you could possibly lose is the last 59 seconds.
2) Too sensitive input stage on the 661. This all depends on your mics and what type of recording you are doing. If you're recording loud music, it is possible to brickwall the 661 even with the pad set to -18. The -18 pad setting is recommended for all occasions unless you need additional gain. On one occasion, I had the 661's -18 pad engaged and still had to engage the -10 pad on my LSD2, a mic that is not all that sensitive as mics go. Some mics don't have a pad (like my Naiant mics), so you might have to add an inline pad which negates some of the smaller size advantage of the 661. Keep this in mind when choosing mics and deck for your intended purposes.
3) The clock of the Fostex matches my camcorder perfectly. This means that I can add an audio track recorded on the Fostex to my DVD authoring software without any modification and there won't be any drift whatsoever. The clock of the 661 started to noticeably drift after about a half-hour. Your camcorder may differ.
A digital input would be nice on the Fostex, but I've managed without one.
The larger size of the Fostex makes stealthing harder, but not impossible
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I have a love/hate relationship with the mic input feature on the Fostex, though. It's great for easily matching levels between channels, but I have had the inputs set too high before. When you do, there is audible distortion until you get the inputs dialed back down (there is an overload light). This is great for situations where you are rehearsing and can balance the mic input and main gain controls, not so great when you've only got one chance to get it right. When in doubt, set it at the 9:00 position and leave it there unless you can't get levels high enough with the main gain control.
As for the dynamic range and mods, this might be a deciding factor if you are recording quiet sources. Have there been any independent measurements of stock and modded units to verify claims? I'd want to see that before spending any additional funds.