Go with the ATI unit. MUCH MUCH MUCH better performance. It's probably more expensive, but definitely worth the money. Keep in mind, what you are looking for in a reef tank setting is PAR...doesn't have anything to do with wattage, or reflectivity or anything like that. You want PAR into your tank.
A couple of things for you to consider.
1.) The ATI fixture has active cooling. Meaning fans to cool the bulbs. Yes, the bulbs. It's been found that fluorescent bulbs have a cold spot. The output of the bulb is maximized for the temperature of that spot. In an effort to maximize efficiency and life from the T5 bulbs, that spot must be kept a certain temperature, something that cannot be done with heat syncs alone. It is also imperative that the lamps be inserted so that the fans provide cooling on that spot...for most bulbs, that spot is found on the label end of the bulb. This means, fans blowing on the center of the bulb, probably doing more harm than good.
2.) Reflectors. German aluminum is aluminum period. It's debatable that it is any better than any other aluminum, and in some cases is actually worse. The ATI fixtures actually have the MIRO coating that increases efficiency of their reflectors many percentage points. Far more than a negligible difference.
3.) Good bulbs are just as important as the fixture and the reflectors, and it is important to have the correct ballast matched with the correct bulb. The ATI Blue + bulb, is somewhere in between the 420 and 460 nm wavelength (i.e. blue/actinic) but puts out more PAR (including at depth) than many offbrand 6500K or even 10000K lamps. (Edit: Most actinic lamps...put ZERO PAR into the tank. They are for LOOK ONLY)
4.) LEDs. These are all the rage and everyone wants them because of the touted electricity savings and the savings from the cost of bulb replacement etc. etc. etc. MOST, not all, but MOST of all of that is hype. Salesmen trying to sell their stuff. We have a local reefclub guy that had money growing on a tree out back that went and upgraded from 6x54 T5 bulbs to LED for his mixed reef 90 gallon...he went with the 3 Watt LEDs. I've seen the tank before and after. Honestly, color difference is negligible in my opinion (with the exception of the "shimmer"). Heat difference, honestly pretty negligible as well since he didn't/doesn't have either one in a canopy where heat is an issue anyway. Lighting bill difference....NADA....yep, ZIP, for going on 3 months now. Maybe a few bucks up and down, but that could be the lights were left on in a bedroom or something. No bulb replacement for 15, 20, 30, years...whatever they are currently spouting. While that may be true, how long with the supporting electronics last? Frankly, none of these fixtures have been around enough to even know how long they will last or how much color shift there will be or PAR loss there will be over time. And how many times to you have to change the T5 bulbs to make up the difference in cash outlay up front? All valid questions in my opinion.
One final note on LEDs. There is starting to be some hobbyists and even some publications that are questioning the ability of LEDs to support SPS and LPS in the long term. Personally I think it's too early to tell what's right and what's wrong, but it's certainly worth a consideration, but I am hearing of hobbyists that switched to LEDs that are going back to the Metal Halides and T5s. Not in huge numbers, but enough that I've taken note.
In short the T5 vs MH vs LED debate could go on and on and on, and I'm sure it will for the forseeable future.
Just wanted to give you some things to think about.
By the way. I can't take credit for all of the information on T5 lighting. There is a gentleman on another well known forum that has spent the last 10 years (he was injured in an automobile accident) researching, testing, taking PAR readings, different fixtures, different bulbs, etc. etc. Unbelievable wealth of knowledge and arguably the T5 expert on the other forum.
If you would like some further reading, please send me a PM and I can send you some links. I've found it VERY useful in my research for my lighting upgrade that will be happening in the next few weeks.
I wanted a kole tang but hay ho.
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/cgi.ebay.co.uk/T5-Aquarium-Lighting-Fish-Tank-Lighting-4-x-39w-90cm-/eBayISAPI.dll?ViewItem&item=310210536929#ht_3561wt_872
After pricing up and talking to my dad i might be able to have a 4 by by 2 foot tank 
Also i can now get up to £120 a month and last year it was £50-70 
