Kcin,
The two Kira products are identical. Boots sell 'UK-franchised' Kira, which is ludicrously expensive and marketed as a one-per-day tablet.
iherb.com sell the 'US-franchised' version of Kira. It is the identical 300mg tablet. However, it is a fraction of the price and carries the correct dosage instructions i.e. three-per-day. I'm afraid it's just another example of rip-off Britain. Even with a three-for-two offer at Boots it still works out considerably more expensive. International shipping from iherb to the UK is around $4.00 (roughly ?2.50) and it arrives in about ten days.
My advice would be to nip down to a branch of Boots and get your first pack of Kira there. If it works well and you decide to keep using it, buy it online from iherb at a fraction of the cost.
In answer to your question, Kira is one of the best brands of SJW and is probably the brand most used in scientific trials. Other brands which use a slighly different extraction / manufacture method and are worthy of consideration are Perika by Nature's Way and New Chapter SC27. Neither of these two brands are available on the UK high street but both are supplied by iherb. All these brands offer something a bit different to each other and are worth a try, but I would start with Kira and see how you get on.
Good luck!
Sheephead.