I wouldn't be the right person to suggest as I haven't been in the market for one in quite a while. I've read that the Garmin Sounder is not as good as the Lowrance but that the GPS on the Garmin can't be beat. For me, a fast responding GPS is critical in order to hit the waypoints exactly when trolling structure. I was on a boat a month ago with a Lowrance GPS and I thought it was terrible. There was a 5 second delay registering our position and trolling was a hassle.
I personally like the GPS/Sounder split/screen since my console is small and don't want two units. But if I had more room, I'd go for separate units so that you can get a full screen view on each. I would suggest surfing around on some of the many sites with message boards dedicated to marine electronics...
178 Sounder
http://www.garmin.com/products/gpsmap178s/
198 Sounder
http://www.garmin.com/products/gpsmap198c/