There's lots of similar questions/arguments on this site regarding R10 vs. R5/R6 but they boil down to "exposure during rebuild". The argument for R10 over R5 is strongest when dealing with the larger, slower disks some buy because their GB/$£€ is better (i.e. 2/3TB 7.2k SATAs) as arrays of these disks can take literally days to rebuild following a disk replacement or addition - meaning the entire array would be lost if a second disk failed during this rebuild window.
For many on this site this risk is too high, myself included. R6 changes this a little but usually brings with it often much slower write performance. Also doing any of this in software further reduces performance during rebuild as all data is going over the same bus, including 'in life' traffic.
You've done a good job of picking your components already and you'll certainly see a huge improvement in performance. If I were you I wouldn't 'fall at the final hurdle' - I'd use R10 knowing you'd done the right thing. If you're concerned about space you can use Thin-Provisioned disks and/or buy the 600GB 10k disks instead of the 146GB 15k disks, the performance drop-off won't be too bad but you'll have a lot more space - you could always buy 4 x 600 today and add 2 more later if you needed the extra spindles?