I am sadly somewhat of a veteran of travelling between Paris and London. I don't know if that's where you are based OP.
Here goes:
Flights - A pain. Longer Check in times, a pain to get to, then having to either take a very expensive cab from the airport, or navigate the RER's. It's a good option from other parts of the UK, but not from London where there is the...
Eurostar - Basically heaven (when it's not on fire/closed/suffering massive delays). If you book sufficiently in advance then you can get a return for £30 or so. Bookings open six months in advance which is when to get the cheapest prices. Most expensive prices for an economy seat are usually around £99 or £130 if you leave it TOTALLY last minute. The check in is minimal, and it takes you to Gare du Nord, which, whilst a complete fucking shithole, is pretty central, and you can easily navigate the metro from there.
Driving - okay if you live closer to Dover, and can be quite fun, but from London it's a bit of a nightmare, and pretty expensive. Not to mention it can be exhausting for the person driving.
Coach - The only reason I would ever recommend someone to take the coach between London and Paris is when: they absolutely cannot afford to take the eurostar, and absolutely have to go to paris. Or if they don't mind selling their soul.
It is the most depressing, dirty, boring, long route possible. A return used to cost me between £16-20. I don't know about from other cities, but from London you don't get dropped off somewhere central. You get dropped off bloody miles away. If you are going just for a weekend, then I assume you are taking the over night coach? I don't even want to talk about how depressing the journey was. It still makes me feel miserable to this day. Often they do pass port inspections for the coaches after the ferry, so you all have to get off, troll through a dingy building to get your passports checked, then pile back onto the bus, which causes further delays. Sometimes, just for fun, both the UK AND French authorities will do this.
If you are really struggling for money at the moment, and this is the only way you can get there, then go with it. Bring some earphones, grit your teeth and try and sleep through most of it. The ferry can be quite nice at night (even if you're normally too exhausted to care). If you aren't counting every penny to get by, then just go and buy some eurostar tickets! If you partner is paying for the hotel and whatnot, then you could always say the transport upgrade is on you, as you want to enjoy more time actually in Paris together.
If your partner had booked a long time in advance, then really the prices between the two transport methods wouldn't be even worth considering the coach.