I have been using the 125 for my 75 g. for 2.5 years now and haven't had any problems. Make sure you get one of the "reef series" models, I didn't and it is a tight squeeze to fit all my pumps etc. in there. I am in the process of building a rubbermaid sump because of this lack of space. Hope this helps.
I've used an Amiracle Pro-Line 2000 for the last year on my 100 gal. reef. Right now I'm converting it to a refugium. I never did use the Bio-balls they shipped with it. Reading here on Reefs cnvinced me to use a deep sand bed (DSB) and Live rock (LR), something I am very grateful for by the way! The DSB/LR combo is super.
If they still ship the Bio-balls, don't use them! Go with DSB and LR.
Also the Rio 600 they ship for the in-sump skimmer burned out after only 11 months and had to be replaced. It didn't do any damage like some of the bigger Rios have done to others tanks, but you may want to keep a close eye on it if you decide to get the Amiracle.
I am also using a Amiracle Pro Line 2000 on my 90 gal reef with a Quit One pump, from my F/O days. When I converted to a reef I just removed the bio-balls. The only negative I have about the system is the skimmer stinks and the Rio 600 died within a year also, but with the bio-balls out there is plenty of room for a real skimmer and heaters.
For a 300 gal tank I think the Pro Line and Sea Reef series might be too small..you may want to look at the Maxi Reef 300.