I have a 70 foot antenna structure that I removed from another farm years ago. Was toying with the idea of putting it up now. The antenna can mount on top of this, or anywhere up and down the structure. With this being said, I would have between 90 and 100 feet of cable. Figuring 100 watts of transmitting power and depending on frequency i would lose 4.5 dB worse case and 1.5dB best case depending on the frequency. I have seen other masts with several antennas mounted on them at different heights? Are they using different antennas for different band widths? Should I be looking to do this as well? I have time to design this and build for the best possible all around antenna. Not sure though how much loss is worth extra height? There has to be a balance? I think I’m rambling now so I will wait and see what members think.
You are right to look at the additional feeder loss as you go higher but 70ft is not mega high. One thing people forget is that depending on the topography, going higher give greater line of sight to the horizon. Depending on the band, you have to factor this in as a maximum - so going up extends the range - even if you get more feeder loss. If your receiver is not severely impacted by the lower level at the input, some signals at the horizon would still be audible. drop the antenna and they won’t be!
Cornstar, you failed to mention what type of radio service you’re considering? GMRS, ham, CB, etc. Please keep in mind, as the operating frequency goes 
, the db coax loss goes as well. LMR 400 coax would be recommended for longer runs to help minimize cable loss.
On the other hand, 27 MHz CB would experience minimal coax loss on a tower of that height.
The answer to your question is dependent on your intended purpose for your 70’ tower?
Best wishes 
WRQE346
I use GMRS to communicate with my kids. We have some very nice GMRS repeaters in my area and they don’t wish to take the hobby any further at this point. I use the ham radio mainly 2 meter. Never considered a CB radio. I understand their popularity is growing again in my area. I’m not sure if you can have enough radios.
Another piece of useful information that I left out. Currently I use a dual band antenna that is mounted in my attic. It would be 25 to 30 feet up. We started with hand held radios and then when I added a base station, this was a great improvement. Now I am rethinking mounting in the attic because of signal loss. Without a doubt, outside would be better.