Selecting, buying and using equipment in this hobby is always a personal decision.
Everything we buy - has an upside and a downside.
I've never used the Emperor 400 - I'm not a big fan of Bio-Wheels.
Plus as Carrie mentioned - having a lot of live rock and live sand will certainly give you enough biological filtration ~ providing you stock the tank accordingly and keep your load down.
Most hang on the back power filters do allow some mechancial filtration though - which is nice to have. These filters need to be maintained and kept clean.
Personally - on a 75 gallon tank - I would take the $80 to $100 bucks - and put the money towards a sump/returnpump/overflow.
Get's your heater and some of your equipment out of view, adds some more water volume to the tankwater system, pulls water from the display tank surface, a great place to do water changes and dose additives and allows you more flexibility on future equipment additions.
Couple of powerheads and you're ready to rock !
One overflow hanging on the back - instead of two bio-wheel filters. With a lot of live rock and a sandbed - I'm not sure how much more biological filtration may be necessary.
Of course - this is my opinion - and what I would prefer to do.
But .... yes they will work if maintained. They are simple to use, do move water past bacteria/filtermedia, add water circulation to the tank ... and if this is your approach - you'll be fine.
