I think saddad's idea of taking both has a lot to recommend it.
Thinking of the brambly site, I know we try to be natural gardeners but brambles are very hard to clear by hand or even mechanically. I had to go down over 2 ft for one this morning, but luckily we have light soil.
It's a good size site so, for at least some of it, I'd suggest taking out the brambles a few inches above ground, then hitting them with SBK. This should clear them but although it's systemic, SBK stays in the soil for six to eight weeks so you'd have to start and maintain some plants off site in order not to miss peak planting season this year or reserve the patch for June/july planting varieties.
You could clear part by machine/hand and get a bit chemical on another bit and see how each turns out - use the best method for the rest later in the year.
