You're pretty heavy so unless that 148 is really stiff, you should prolly go a little longer, like 152+ at least, especially if you want to basically free-ride. If you're mostly into riding fast, I'd go for the stiffer board. You will not have as much control over it for now because you are not that good yet, but as you improve and get more comfortable at high speeds, you will futureproof your setup. In general, if you're not into the "park" and are aiming for speed, just go stiffer...unless you don't really snowboard that much that it will take you a long time to get to that level. That's your call whether you want to try medium first. And if you get "really good", a stiffer board is better for long jumps too in the park. Though normally, the recommended progression is to start small. But heck, if you're so fast and fly over declines at 40 mph and land perfectly...it's not much of a difference going over 60 foot booters.
If you like ice, look into "magnetraction". I have one of those boards and it's pretty good. Like my park RC (reverse camber) board is better on icy terrain than my stiff C board and RC and soft usally means crappy edge hold. But magnetraction smokes the ice.
Also, I'm about your height but weigh much less than you and shoppies had recommened that I get a 156 because I told them I can do basically any double black diamond run. So they prolly said that because they didn't think you'd be going that fast or whatever at your level. But I recently got a 148 anyway because I going for park and I don't really like to go fast. I feel I'm wasting my ticket if I get down that fast and it's not that "fun" vs. freestyle for me anyways.