The best grease is the grease with the least amount of contaminants. Everything else is secondary to that.
Naturally grease does not come with contaminants in it, but, in the workshop it attracts any dirt going, as if it were some huge magnet.
Therefore, packaging is important. The large tub that you have for the car/motorbike/boat/unicorn cage might not be too well suited to the bike because the small quantities needed for re-packing your hubs might be hard to cleanly dispense. Hence the small tubes, as sold in bike shops, are worth the money even if you already have a vat of the stuff at home. With a tube you can always wipe the end.
There are affordable tubes of grease, e.g. the 'Weldtite' line of 'lithium' grease in the UK, these work absolutely fine and have plenty even in the smallest tubes. There are also more expensive grease tubes with fancy dispensers and fantastic formulations - imaginably these are vastly superior but, in practice, the grease is secondary to pre-load, part wear and correct assembly.
If you had two identical bikes, one packed with the posh grease and the other with the affordable stuff and did some 'blind riding' I doubt you could taste the difference or find that you would need to service one earlier than the other.
On the other hand, if you had a bike refurbished by an experienced mechanic with normal light lithium grease and an identical bike refurbished full of posh marine grease by someone that has to think twice about which way to tighten a bolt then I think the former would have noticeable ride quality.
Sometimes a purchase is of psychological value. I would prefer to have my Campagnolo parts with Campagnolo grease, my Shimano kit with the 'correct' Shimano grease and not feel 'let down' by cheap grease. The affordable lithium grease is light (which is correct) but one imagines it to wash away.
As for your application of the seatpost, the stuff you have will probably work fine, so long as your seatpost is not carbon fibre or your frame not be titanium. Otherwise, you must grease your seatpost unless you want to use the blowtorch to get it out again. The best grease for the seatpost is the light lithium stuff - a light coating of the affordable stuff wins because you don't have uneven distribution or it too greased (needing the seatpin bolt to be massively tight). A third benefit is that you can clean the excess a bit easier.