The estimate of 14-16 days & 20-25k for training and implementation is REALLY overstated to go to macola 7.6x. You could easily convert from 6.2 to 7.6 & get training for under 10k (btrieve), especially if you have a dedicated conversion team in house. That $3500 10 user upgrade can be used for either btrieve or sql. You will have additional conversion costs if you go mssql, maybe another 3-5k, but you will be positioned for the next generation macola ES. If you start btrieve & then go SQL you will have to pay another $350/user at that time.
Netcellent does pretty well for a small shop & they are pretty good at integrating with other products like shipping systems, edi, etc. I would question how well they will do long term, however, with the consolidation going on in the software world. Did you just see the news that Sage/Best bought accpac? They are bigger than MBS now. A 6.2 to NSI 7x conversion, training, implementation is going to run about the same as a Macola 7x btrieve upgrade.
Whatever way you go, you are looking at getting a new server or servers & the necessary db manager to operate the software. Time to start spending some money.