Eiffel Tower

I've done both , many times for ET , just twice for Monparnasse .

Honestly I do t think you can compare them . Sure both have great views , and Monoarnasse has shorter or no lines , but there is no romance there , no sense of history .

However both will give you great views , so I can't push one over the other if just looking at it pragmatically , shorter lines and great views , Monparnasse , but for a sense of ooh lala I think the ET has that well over the Monparnasse

The lines to walk up the stairs at the Eiffel Tower are fairly short. It's taking the elevator back down that is time consuming.

If you're just going for the views rather than a "tower experience," consider going to the Trocadero view point (free, has sensational views of the Eiffel Tower) and/or the top of the Arc de Triomphe (covered by Museum Pass, great views of the whole city, as well as the entertainment of watching the cars play dodge-em around the Arc). I did each of these several times, by day and by night, and when I go back to Paris, I plan to do them again (whereas I've done one trip up the Eiffel Tower and don't feel an urge to go again, particularly given the crowds). I haven't been up the Montparnasse Tower.

Old joke, told by architectural connoisseurs and snobs:
"The best thing about the view from the Montparnasse Tower is that you cannot see the Montparnasse tower."

We've done all three -- Montparnasse, Arc de Triomphe, and the Eiffel Tower-- and all were stunning views. We thought the ET was magical but we ascended first thing in the morning before they had the timed entries and the crowds were not so bad. Montparnasse was even higher and gave a great view of the most notable monuments, the Arc and the ET. The Arc de Triomphe would give a marvelous view, especially of the Christmas lights down the 1.2 mile length of the Champs-Elysees. We just might take in all three again when we are there in mid-December. If you can't do both, then by all means make it up the Eiffel Tower, the very symbol of Paris.

If you're just going for the views rather than a "tower experience,"
consider going to the Trocadero view point (free, has sensational
views of the Eiffel Tower) and/or the top of the Arc de Triomphe
(covered by Museum Pass, great views of the whole city, as well as the
entertainment of watching the cars play dodge-em around the Arc).

We choose Harold's method as well and were never sorry for skipping the top of either. The night view of the Tower from the Trocadero is wonderful as was the panorama from the Arc de Triomphe. If you have the Museum Pass, the Towers of Notre Dame is pretty great too, and getting up close with the Chimera is a plus!

I've had the pleasure of views from the top of the Tour Eiffel, Tour Montparnasse, and the Arc de Triomphe, and from the Trocadero and La Defense, and enjoyed all. I would do so again.
Eiffel: great view of the city and close-up views of the tower structure. And, why go to Paris and not go here?
Montparnasse: great view of ALL the top landmarks, including the Tour Eiffel. No line, no wait, quite windy.
The Trocadero has an excellent closer view of the Tour Eiffel, but is not as high as the others for full city views.
The Arc de Triomphe has an excellent view of the beautiful Champs-Elysées, as well as a nice view of the city.
La Defense: business district out of the way for most tourists but very easy by métro. Distant view of all other top landmarks.

I'd also add Scare Coeur hill in Montmarte to the list of great Paris viewpoints. During the day, you can tour the cathedral, and on summer evenings, it's got a certain street party feel (just secure your bags like you would anywhere in Paris) If you can't or don't want to walk up the hill, you can also take the funicular up- I think it's one Paris Metro carnet ticket to use that hill train.

Thanks everyone for the great advice!

The Eiffel is 1063 feet, Montparnasse is 689. Southam's observation is right, the view from Montparnasse includes Eiffel, and vice versa. Montparnasse is easier to get to the top, less crowded, comparable view though not as much of the river. You don't need advance tickets for Montparnasse, so bad weather is less of a risk -- plus it has an inside viewing area with big windows. So I'd recommend Montparnasse.

That said, you will want to see the Eiffel from close up as well as from someplace like Trocadero. If you'll be there anyway and time is short, maybe it's a better bet. I never tried going up the stairs but, if you're up to it, that could be one way to go. Consider going up to the first or second level for a good view of the scenery and the structure itself.

You do not need to pay museum admission to eat at the cafe in the architecture museum at the Trocadero, and the cafe has a really awesome view of the Eiffel Tower. I think one of the least hectic and crowded parts of our time in Paris was sitting in front of their big glass windows and enjoying the view over coffee and macarons while not having to contend with the petition girls and trinket sellers just outside of there.