I've already replied to Lisa as asked via PM, but to ensure others have the benefit (or not, whatever
) of what I suggested...
Eheim have a proven track record and they are certainly good filters from operating, efficiency and effectiveness. I also have a Rena on one tank, and it from what I can tell is as effective and efficient; the durability is somewhat unknown and can't be compared to Eheim that has been around for many decades.
I personally prefer the heating unit in the filter, as it is with the Eheim on my 90g. You would need two heaters in a 90g, and 200+ watts each.
I have two heaters in my 115g, placed one next to the filter outflow and one next to the filter intake. On an average day with constant room temperature (i.e., not during a summer heatwave) the temperature in that tank fluctuates very minimally; according to the electric digital thermometer it sometimes varies by .5 of a degree; this is measured at the outflow from the filter so I know the temperature of the water coming out of the filter and entering the tank. Sometimes it is .5 above the set temp (25.5C/78F), sometimes .5 below--in other words there is a variance of 1 degree C which is 1.8 degrees F. Not a problem, but worth noting.
On the 90g with the Eheim and heating unit, the thermometer probe is next to the filter intake so at the opposite end of the tank from the filter outflow; the electric digital thermometer display indicates the temperature fluctuates by .1 degree C from seconds to seconds; rarely is it .2, and even more rarely .3 throughout the day. In other words, the temperature in this tank at the probe is fluctuating by .1 of a degree or at most .3 of a degree.
I have had the Eheim with heating unit running continuously for more than 12 years, and there has never been any variation. It is certainly reliable. I have had more than one hang-in heater malfunction, and lost fish one night when it overheated the tank. But in fairness, the high-end heaters in the 115g have (so far) never failed, and they are 7-8 years old.
Briefly on cost. In Canada I can get the Eheim with heating unit for $300 on line. The basic Eheim is $175-200 and reliable heaters would run close to $75+ each. Even on sale, there is little difference.