Aside from the attic topic, the decision on Honeywell vs Elk depends on the features you're looking for and the level of monitoring service you're interested in getting (if any).
A Honeywell VISTA-21iP would require an AlarmNet account if you wanted to use the IP communicator for monitoring and you'd get that service from an alarm dealer that offers AlarmNet monitoring (most do). You could have it just central station monitored and/or you could get Total Connect 2.0 service so you could have remote access from a smart phone or computer and text/email alerts from the system. You could also add a cellular communicator if you wanted cellular back up on your monitoring service in the event the internet path is unavailable.
The 21iP can support 8 wired zones (15 with more complicate zone doubling) right out of the box and with wired and/or wireless expansion modules, you can expand up to 48 total zones (zones 9-48 can be wired or wireless depending on which receiver(s) you use). The Honeywell 5800 Series sensors work with a VISTA-21iP that has an appropriate wireless receiver. A 6160RF is a wired keypad with an integrated wireless receiver that would unlock the full capability to program up to 40 wireless zones as needed. It's a nice option if you planned to use a 6160 keypad and a wireless receiver anyways. There are other receiver options as well.
I'm not as familiar with the Elk but most of the people on this forum use an Elk over a VISTA so I'm sure others will chime in.