ex3 video, mixed with beta SX, whats best
 

is that possible to make them splice together ok? if I downconvert to 4:3 with clip browser as I am transfering the files, is that all I would need to do?

It will work, but it will re-compress the footage to DV.

What system will the footage be edited on?

vegas 8c. yeah, the video will be DV, but so is the beta sx video I will be working with. I want to use the ex3 for some timelapses and slo mo stuff, but am using SX for the client. and want to make sure its all going to look close enough in the end.

Well you should keep in mind that SX is actualy 4:2:2 SD so converting the EX3 footage to 4:1:1 DV (with clip browser) is not really the best thing you could do.
4:2:0 HD (aka XDCAM EX) actualy provides you with fairly enough chroma data for a very decent 4:2:2 SD footage, so you might just simply consider using the full resolution EX3 footages rather than first downsampling them to DV, given of course your NLE can handle the full rez footages. If not you could also try to figure a way to convert the 4:2:0 35mbps HD to something like a 50mbps 4:2:2 SD or just uncompressed SD (via sofware or via the SDI input).

That said, I'm not really sure the color sampling will be the most visible difference between both footages. SX camcorders are typicaly based on 2/3" CCD chips with older DSP processings whereas EX camcorders are based on 1/2" CMOS sensors with fairly new tech processings (including hypergamma if selected). Without arguing on pros and cons of both technologies, it might be a tad difficult to adjust them in order to create pictures that actualy match when displayed onscreen. I suggest you run some tests with both cameras to see if the pictures are convincing enough before planning the shooting.

so maybe just put both pieces of video on a SD timeline in their native form(with the HD video squeezed or edgecropped) instead of downconverting?

Obviously edge cropped would be the best option if the final product will be full 4:3. If your NLE handles different resolutions on the same timeline, I guess it should provide you the highest quality possible for an SD output.

Do you actualy have both cameras around ? Have you tried to look at both sources edited together in a sequence ?

yeah, as far as I know, I should be fine editing with both formats on the same timeline. I THINK.

I wont have both cameras till march, but I will be testing them a lot before the shoot starts to see what I get.