Yoho, What size of fish are you hunting? I used to have an RA Sparid and it has many features that I like, not least flexibility (you can add a second band or switch to a 20mm rubber) and robustness (exceptionally strong). It is quite big and heavy gun by modern standards. If you are going for a lot of big fish and need double-bands go for the Tuna (but with 2 rubbers & a thicker spear it will be heavier and take longer to load) but I found the Sparid more than powerful enough for the UK fish (I tried 1x20mm rubbers for several years but came to the conclusion that I prefer the 16mm rubber - 16mm bulk rubber is much more powerful than 16mm or even 18mm screw-in bands and is much pleasanter to use than 20mm). 2x14mm is another option to consider (popular in Hawaii).
The Scorpia is probably a perfectly good speargun and probably more than good enough for most spearos most of the time but, I just find it unexciting and it looked like they'd cut too many corners last time I looked at it (the black and white photos are probably intended to convey that feeling!
). I think it risks devaluing the RA brand - it certainly doesn't have all of the features I expect from RA. Would prefer to see them lower the price of the Sparid & Tuna and bring the carbon barreled models more to the fore. A speargun can last you decades, so if you can afford it, I would go for the Sparid over the Scorpia - it will be easier to resell too (I sold mine on) but if you can't or don't want to spend so much, get a Scorpia (. or Omersub or Beuchat), it'll be fine. The RA lifetime warranty is worth having and tells you a lot about Rob Allen and his company.
The Beuchat Mundial Oceania looks like a copy of the SA railgun - nothing wrong with that & Beuchat products I own seem good to me (except their crappy lightweight muzzle bungee - not a problem on the Oceania). I like the open/close muzzle feature - the video below is wrong, this nice feature has been available on some Omersub models for years (e.g. XXV & several Cayman models)
BTW Visibility and therefore speargun length is far more important than the actual model of speargun -- so whatever you buy, make sure you get a suitable length. If conditions vary a lot, you might need 2 spearguns, 1 short and 1 long.