actually ya can use a harbor freight in exactly the same way as the portaband saws I do it all the time and I have both available, as far as cutting curves ya can't cut any tighter with the portaband than ya can the 4x6 in its vertical position, the blade for metal cutting saws are wider than those for wood and none of em are really great for cutting curves, do a google search for
4x6 metal cutting bandsaw
and ya'll find a ton of tables and so on people build for em to use in the vertical mode and still have the ability to go horizontal.. there is a pin on the newer ones to lock em in the vertical position just cause its so popular...
the reason I asked the guy above "really" regarding him saying the HF 4x6 is notorious for blades jumping off is because if ya do a search ya will find thats not correct what they are most notorious for is the versatility... I have several portabands most are Milwaukee 1 is dewalt they are onea my favorite cutters especially if I'm cutting miters on the horizontal saw cause if its off just a hair its fast to just slip the porta band in there and trim a blade width off without straightening the joint etc... but as far as Bandsaws go.. for your application ya will probably do better with a 4x6 if ya can only get 1... orr check craigslist and offerup look for a good deal on a 4x6 like the one I picked up for $30 it just needed a lil cleaning I originally grabbed it as a parts doner for my old 1987 one but soon realized it could run and it has ever since... then buy ya onea the NEW Milwaukee portabands (I've got em back to the 6 amp American made 6030 up to a brand spankin new cordless one it or its corded cousin are the best Milwaukee has ever made! I have both versions) tip the brand new Milwaukee portabands pop up on offerup all the time for around $150 in unopened cases etc... then ya will have both options available with a bunch of your budget left over to go towards a china plasma cutter...
EDIT; BTW as far as how fast the 4x6s cut is totally dependent on your blade choice... cheap blades are unbearably slow a good blade pretty fast I had a new harbor freight bi-metal blade someone gave me so just for chuckles I put it on the $30 saw and started both saws (the 1987 saw had an old blade that had probably 300-350 cuts through 2x2x1/4 and 2x2x 1/8 on it) set at same speed cutting the exact same 2x2x1/8" square tube.. the Olsen blade ($26 on Amazon if ya only buy 1 I buy em 5 at a time) was done while the HF blade was only 3/8" through its piece... Sterrette blades for me cut a lil faster than the Olsens but not enough to justify the extra $10 your blade choice will also determine how straight your saw cuts... and your blade setup will determine how long your blades last