Must pull trigger-- quick arm help please

Is there no chance you can listen to the arm/tt combination? If you really, really, have to choose in a hurry, I would go for the 300, which is structural better IMHO, despite the tweaks on the 250. But if you have a chance, listen for yourself!!!!

As Satch explained, it's your call. But, since you asked: Incognito wired RB-300 with Kerry F-2 Titanium Heavyweight Counterweight still gets my vote. This counterweight is just too good to miss.
My Kerry-weighted, standard wired RB-300 still provides pleasure to match some much more expensive arms in more extravagant systems to which I have listened. And, you will do even better due to your Incognito wire upgrade.
You might mention which cartridge(s) you are considering, so the RB-300 faithful can weigh in with their suggestions.

Thanks.
Really no possibility of audition.
Dyna 10x5 is the cart I will use, and Nott interspace the table.

But I cannot swing both the rewired RB300 and the Kerry counterweight at the same time.

I'd pick the modded 250.

Read the info on the OL website, on the 250 vs the 300, and how they each respond to mods.

I second the Incognito RB250. You can get a Mitchell technoweight latter for that arm, an easy 5 minute change. I just bought one of these arms and I'm loving it.

The 250 has better things added onto the basic arm. The bearings on the 300 were originally better than the 250, but by the time OL did the mods you are coming out ahead. Besides, TWL said to go with the 250.

The 250 also accepts TWL's HIFI mod and my VTF on-the-fly mod easier than the 300. Maybe not a big issue with a 10x5, but certainly a huge issue if there's ever a chance of using a lower compliance cartridge. I vote 250.