Having just purchased a new (2018) MBPro build to order with i7, 16Gb RAM & 1Tb drive, I can assure you the speed difference over the older Thunderbolt MBP with removable SSDs is significant!
I like you, was upset when Apple started to solder the RAM & now SSD to the motherboard, but as long as you spec the best you can afford, I don’t think you’ll be sorry.
Just as a guide, I bought a Samsung 1Tb SSD as a backup, and via Carbon Copy cloner, it only took 50 mins to backup 500+ of data, whereas before, it would take hours over Thunderbolt, and a day over FireWire!
Enjoy your new MacBook whichever way you decide to go.
Cheers
Mike R