Another vote for a Soft Ark if one is purely looking for a mid-grit natural, though with some caveats...
Firstly, they're a lot more effective on some stainless alloys than others... For example, excellent on the softer Euro stainless knives that
@Benuser mentions, good on some Ginsanko & AEB-L's depending upon the hardness, basically useless on R2/SG-2, etc. Does well on softer carbons (Nice for some of the cheaper Chinese cleavers, which aren't always made from the nicest of steel, or heat treated especially well.), or vintage carbon butcher knives, but pretty useless on Aogami Super, etc; especially at higher hardness... They're sort of in a 1.5-3K ish sort of range, depending upon how they're lapped, how burnished they are, etc. Like all Arks, the edges off of them are very 'crisp' compared to synthetics, or even some other naturals.
Unlike the harder Arks, these will dish a bit in normal use, but are a bit easier to lap (Still not fun to deal with a heavily dished one.). Definitely use coarse SiC grit or sandpaper to flatten them, rather than a diamond plate. If used without oil, they will clog/glaze and need re-lapped much more often (I also find playing with oil thickness can be useful in modifying how the stone cuts.). They also are still much harder and less forgiving of sloppy angles than synths, let alone BBW's/Coticules or many J-Nats, and are slow cutters for their grit compared to synths or even some of the low-grit J-Nats. This results in excellent edges (Little or no slurry dulling/edge rounding with a steady hand, and good control of burr formation with some patience.).
That said, their 'deliberateness', combined with how beneficial it is to use them with oil rather than dry, doesn't immediately lend itself in my head to 'kitchen touchup stone'.
Again, as Benuser said, specifically what sorts of steels aren't holding an edge well enough for you to get through the prep session? For the softer, lower-carbide ones (Up to 60-61hrc.) F.Dick honing steels can be an excellent solution for a variety of reasons.
If an edge fails on me mid-prep, and I feel the need to resharpen it on a conventional stone, that means I need to drop everything I'm doing, wash the knife, try to put an edge on when I'm distracted and under a time constraint (Never with as good of a result as normal.), wash the knife again, and go back to cutting... I hate this.
If the knives you are using are responsive to it, steel-honing on a good quality steel is a great solution for mid-workflow touch-ups, as it removes minimal material to restore the edge, it creates little to no mess, and the rod can be sterilized after so you don't have to wash the knife before/after honing it. You're out of the game for minimal time, with minimal material removal. There's a knack to learning how to steel, but if you often use knives that are responsive to it, it's worth picking up a good rod or two, and learning. F.Dick Micro's are great for higher-quality, thinner-geometry knives. F.Dick Classic/Sapphire Cut is good for somewhat thicker-geometry, slightly lower quality ones (Soft Euro stainless, cheaper carbon Chinese cleavers, etc.).
If you're needing to touch-up high-hardness/hard-carbide steels that
aren't responsive to steeling, mid-workflow, it's trickier... In such situations, I'd often rather wash/dry, and pick up a fresh knife than try to sharpen mid-workflow...
However, if you are genuinely cool with stopping your workflow to sharpen, and wish to go natural...
Quite frankly, your BBW/Coti with a bit of slurry will easily match or exceed the speed of a Soft Ark, on a wider variety of steel alloys... Edge feel is a bit different, but depending upon the steel, even finishing on some slurry with these stones isn't always bad. Jumping from an SG500 to either a soft Ark, or a BBW/Coti on mud is typically a bit much, if you're looking at this sort of progression. 800-1K, sure. 500-600, that's pretty cuspy unless you've got a crazy-fast Coti on full mud.
Going a bit coarser, one of Maksim's (Japanese Natural Stones) White Binsui's is a decent choice if looking for a splash & go natural that responds more like how the J-Nats or synths most people are used to do... This could pick up from an SG500. It needs either re-lapped frequently, or slurried, as it does tend to load and slow down fairly quickly (Which makes it finer. Also an interesting stone to play with finer nagura on.). However, I generally agree with Maksim's 1-2K grit assessment, and can attest that this is a nice transitional stone for kasumi work... On edge bevels, it does suffer more convexing/slurry dulling, dishes more, and even when fully glazed, it never creates as fine/crisp of an edge as a Soft Ark. However, it is faster if used correctly, it will pick up from an SG500 easily, it's useful in a natural polishing progression below your Aizu (Arks are rubbish polishing/thinning stones.), the edge is just a bit better feeling to cut with than a 1K synth (Though the synth will work on more steels.), and it's a big, beautiful, splash & go house-brick of a stone... Just like the Ark, though, it's best on simple carbon/stainless alloys.
Hope this helps.
so something under $100 would be better. I like the idea of a soft ark or washita, never tried one. 
